Could you elaborate ? I am having a hard time seeing the logic, Substance of Shadows break with any light, it is very good but also situational, any situational, is is either useless or makes the problem a non-insue so I don't see why it would factor it, and again while dispeling is something we are awesome at a situation where we are focused on that isn't one where we would be using Speed of Light, and if dispeling could help during one turn our equipament tend to have it reasonably covered.
My problem with area of effect spells is that they tend to be indiscriminate and we tend to find ourselves in oh shit situation during infiltration so chances to use it that way would be rare, on the other hand I think people forget how ridiculous our sword is, with speed of light we would have almost maximun killiness in a controlable away witch would be more flexible to use.
I like Substance of Shadow a lot- whether as a getaway or infiltration spell- but I admit it's not the most critical piece. That honor goes to counterspelling.
If you look at our confrontation with Alkharad, you'll notice that he was throwing both spells and physical attacks at us, and Mathilde was using both physical defenses and active magical defenses. Here's the relevant bit, where he uses
dhar eye lasers and Mathilde counterspells.
[Mathilde vs Alkharad 4: Learning, 83+26=109 vs 76+34=110]
A moment before it's too late, you spot the stirring of Dhar within the vortex of it that a Vampire resembles to Magesight, and with an effort of will deflect the projectiles that fly from Alkharad's eyes, spinning off into the room behind you...
[Mathilde vs Alkharad 5: Martial, 57+23=80 vs 11+28=39]
...and effortlessly cleave through Vampiric flesh to send the other arm flying, taking the energies of the second spell Alkharad was shaping with it. You look at him as he looks down at himself, and he sighs.
Now, admittedly if Mathilde is high on
hysh she'll be doing a heck of a lot more swording and at least in theory be better able to take down enemies before they can cast more spells at her, as in her second "I counterspell with my sword" bit there, but it seems to me that if we need to call on Speed of Light to take down an enemy, they might well be potent enough that they'll be able to get a spell off in the face of it. And when they do, I want her to be able to smack their spell down so that she can sword them to death, as she did for the first spell. If she's unable to cast she'll also be unable to counterspell, because all Mathilde's magical actions are dependent upon the use of
ulgu.
She does have the belt... but as this confrontation again demonstrates, that could very well be expended by the time Mathilde is pushed to the point of burning her emergency battle spell. We can't count on it being available.
As for AoE vs. personal buffs, mostly it comes down to a use case. I feel that Mathilde is already very deadly in combat and turning her from very deadly to absurdly deadly for a short while isn't likely to solve any problems she runs into- generally, they're problems where personal prowess isn't enough to solve them, like "there are literally hundreds of enemies trying to kill you; you can kill one every second and it doesn't matter". In those situations, there's a chance that an AoE can flip the table and give her an opening to succeed in her mission or escape. Likewise, Mathilde can already assassinate most individual targets successfully; Speed of Light would make her even better at 1v1 duels. But a good AoE could open up entire new vistas of targets she could kill out of nowhere, like "the entire enemy warband" or "this small fortress", because again, it fills a niche that she does not currently have available to her.