This post is a bit nerdy and involves some amateur science and engineering though hopefully number light and understandable.
TL;DR: I think you can get around those issues. But the real question is would you find it useful if the techs were split up and you could get some of the benefit cheaper (and thus possibly earlier)?
Lasers don't get in... but neither does any other part of the EM spectrum, which means that your sensors don't work. This is why we need the gravimetric sensors tech before we can make any practical use of TIR shields.
This is (mostly) true. TIR leaves a tiny hole that get increasingly smaller the more powerful the field. For example a 5000c field*, covering a 20m radius sphere (the front of a frigate?), leaves a hole with a 4mm radius. Ironically the center of the hole is always located at the place where a line from you to the field would make a right angle with the field (yay light geometry!). Thing is UberJJK has mentioned before and I can see his reasoning that said holes could be exploited to give vision assuming they aren't too small. Thus some sight at FTL at least. Careful creation of TIR barrier geometry and camera placement may produce an odd, limited, but still (hopefully) useful picture. On the other hand the attacker at lets say 10km needs to be able to line up a shot with 0.000046 degree accuracy. And it just gets worse further out. I'm not sure of the limits, best I'd heard is ~0.00014 (amusingly from a company called PI), for medical scanners.
*which we can be 100% sure at least some ME ships can make, though some other estimates give much larger numbers 110,000c for example is the one I use for the maximum of normal Citadel tech. That hole is 0.18 millimeters in radius. (0.0000021 Degrees accuracy at 10km)
TIR isn't perfect, it's just
really damn good. And when what the enemy need to get around that is something they don't have? Then really damn good is good enough.
Now double layering should hopefully close those holes, but covering them thus making TIR stealth work. I'm not going to claim to be a geometry expert here, but Yog claims the leak should be small enough for stealth. I think that after enough time the amount of energy in the "gap" will get to high and even that tiny leak would still be an issue. However I'm not sure how to approach this issue mathematically.
EM can't pass through in either direction, which means you are sitting inside a giant perfect mirror, just you... and the fusion/antimatter plasma torch you're using to maneuver. The enemy won't need to roast you with gamma rays; you'll be doing that just fine by yourself. This is why we need the Repulsor--or I guess the Normandy's weird gravity-based Tantalus drive--before using TIR laser-shields.
With a continuously active thin layer shield this is an issue. The field will reflect emissions back, as the ship is in a low C zone and is surrounded by a high C one. You'd need some way to drastically reduce heat output (eg Reactor + Repulsors).
However I sort of put a joke into my observation about FTL systems. "You can after all jury-rig one using an FTL drive, a bit of know how and about
5-10 seconds of work." Those 5-10 second are literally only for the time it takes to turn the thing on. (Well plus or minus boot up time and any safeties, but w/e) I wasn't exaggerating for comedic effect. Having an FTL field covering the ship provides a TIR field meaning the Jury-Rigged version is actually immune to this problem. The emissions begin inside the high-speed field and thus aren't blocked. Its only out-field to in-field that's blocked for a high C field not the reverse.
So yeah those are less of an issue then you'd think. Now draining you FTL time maybe an issue though that depend on engagement length. Also the question of shield-FTL compatibility.
Spit balling some engineering solutions for the TIR Shields tech here:
The first problem is an issue anywhere you can see they can shoot, so barring other work arounds, (Grav-wave sensors, QEC probes, and maybe Other things?) the question becomes, is a a hole large enough for sensors an issue and can you mitigate that? (See above) Also should be noted that you only need to cover valid vectors for a laser shot not the entire ship. That's a case of the detection game though and losing is (possibly) lethal.
If you're really crazy you might be able to pulse the field (or its intensity at least) and get snap shots visuals. Depending on a lot of factors including pulse rate and camera ability, this could at least reduce laser damage by a significant margin (for long/many pulses) if not out right block most (or all) of the shots (Shorter pulses). Then you need to fire laser shots in turn with the targets shield frequency. Which sound very startreky.

Of course you might be just producing a warp (no not the startek one the ME one) so I'm not sure that'd fly for an effect that was in contact with the sip.
The second you can possibly get around by doing funny things to shield geometry. Buy hugging the shields and having the Radiators be (partially?) inside the field they can still emit safely. The Drive torch maybe a complicated issue most of the heat from that come from inside the ship, after its been shot out (and past a hugging field) that's less of an issue. The internal heat would need to go to a radiator anyway. So... Meh.
Also if you can get away with partial coverage...
My thought would be that you institute a partial coverage scheme, and project the field away from the ship in a thin layer. Then to keep an eye on the enemy so you can only cover the right vectors you'd pulse the shield, assuming (quite reasonably I'd think) that getting good combat visual coverage using the right angle holes is an issue. You'd then vent you heat out the other side. Down side is that it not perfect or even nearly perfect and if you're surrounded you start building up heat like a no tomorrow.
The other solution of course is to just have a full ship TIR (aka FTL field) and some really complicated camera system thingy, whatever-you-call-it.
My real question since a TIR stealth system is a two layer system and a TIR shield only needs one layer (either a thin one if other think that'd fly or full ship one if not), if I split it into two techs, with TIR shield being IDK some thing like this:
TIR Shields [400-600?]: By altering the speed of light Mass Effect FTL fields bend the path of the light. Mathematically this is identical to refraction in light traveling though a medium, and some theories conclude that the why behind this is the same. Regardless if the speed of light in the two volumes of space is too different an event known as Total Internal Reflection (TIR) occurs. Using a TIR event light can be reflected protecting a ship from lasers and other photonic weapons. Unfortunately this leads to issues with a ship's sensors and heat build up, tough other technologies may be able to compensate if they can't be solved tough cleaver engineering. Unfortunately this will be incompatible with other active shields, though that doesn't prohibit multiple shields being installed, merely requiring only one be used at a time.
and then had TIR stealth requiring it and being discounted by the price of TIR Shields, would that influence any decision making on when to pick up TIR shields/Stealth? Maybe buying it earlier for fighters/missiles? If you'd still get it at the same time then it doesn't matter (well there's a one quarter gap but ignoring that), but if you'd pick it up earlier then it maybe useful to you to split (which is the only reason I'm suggesting the split). Stealth in space and fuck lasers (mostly), are both destinct enought that they can be ther on techs.
Of course the engineering (and additional techs?) involved in making it work properly are still up for debate too.
a good reason that the Citadel races aren't already using TIR shields.
(Partial?) Blindness sucks, the safeties are a pain (for FTL hack), and no one uses major combat lasers anyway? (And since you can only have one shield on at a time you pick the one that block the worst weapons?) Personal watsonian head canon tells me that the no one use combat lasers thing and the no one use TIR shields thing are sort of a chicken and the egg deal in military tactics. (FTL came first

) (Of course there's a Economic/Efficiency argument and other bits too) Of course Doylisticly it's that the writer(s) didn't envision the advances in laser tech and/or setting style choice.
But that's just sort of my try at explaining mass effect canon when it makes no sense. (Now to contemplate why a cheap barrier is enough to stop an FTL strike!

)