My argument for Tech Level 10:
To preface, yes, there will be a diplomatic shitstorm with -3 Relations to -4 Relations (Muspelheim and Niflheim probably won't care/care less*, Byzantium will care less**, Vanaheim/Bertil will care a lot) to go around, and Rotbart will gain a reputation as a Bad Deal-Maker. But it is absolutely worth it.
So, what exactly can we do with -20% time reduction that is so incredible?
Well, the must-do actions of Telepathica Construction would have had 4 years saved in the past, 3 years (and more) in the future. That's enough time to do one piece of major research. Upgrading Dis would have taken 21 instead of 26 years. All around there are major time reductions for projects, which then translate into basically having more stuff done. On average in a 5 year turn we'll save enough time to do a Large City to Small Hive upgrade. It's not the flashiest of benefits, boring but practical but it's these kind of benefits which we can use to basically do more stuff.
Think of it like this: It is strategic-level Chronomancy minus the warp backlash.
There's also side benefits. Daemonic Incursions are unlikely but inevitable events. Whenever they happen, everyone gets invaded. Now, Trolls with TL 10? They'll handle a lot more Daemons than us - And Trolls fighting daemons are Daemons we don't have to fight.
And the negatives.
First, being known as a bad-deal maker. In a more sane galaxy this would indeed suck and suck a lot. But, ah, there are very few people to actually make strategic deals with. Apart from Dragon's Nest, which we are merging with, there are literally no nearby polities to make a deal with. The Conclave? Permanently set on automatic free tech sharing. Eldar? Favors we'll get through Rids pretty much, and in their usage we're trusted enough as a General that we can call in them to get Military support when we actually need it.
Other planets? I guess they could well try to sucker-deal us in general economic, Vanaheim in specific. But that comes with non-trivial risks and can actually result in majorly screwing themselves over.
Plus, let's remember that materially, this practically a pure-Avernus consequence decision. Like, seriously. The Trolls don't want to leave the planet, and if anything goes wrong, the wrong will be purely and entirely focused on Avernus the colony. Literally null backlash to the other planets. Trolls aren't going to invade them. It's similar to our governmental decisions.
I mean we've already eaten massive diplomatic consequences for trading souls to sirens. But the saddest part of A Ruthless Reputation is how it applies so rarely due to everyone applicable being dead/corrupted or literally too far away.
*Note: They are basically giving AM/EM for free to the colonies.
**Note: From outside the trust and have gone through all the shit in the galaxy.