On the one hand I can understand removing agility quirks because to engine decoupling, that's sort of alright. It's a nerf to some of the best clan mechs relative to the STD engine IS mechs. So when comparing KDK-3 and TBR to mechs like STK-4N, MAL-MX90, WHM-BW or GRF-3M then I guess we're moving in the right direction. Note though that the NGR will not be affected as much so the powercreep-o-meter is still stable.
On the other hand, that last part is important. There is a family of IS builds (including many of the few viable IS builds) that are viable only because great agility and great shielding arms/hitboxes allow them to perform decently with an XL engine, which in turn allows them to carry a viable weapons loadout. I am thinking BLR-2C, BLR-1G, BNC-3M, WHM-6D, GHR-5P, BL-KNT, QKD-5K, SHD-2H etc. I expect these to take a really bit hit in performance when they can't twist fast enough housing an insta-death IS XL engine....
So, only considering the agility pre-nerfs I feel that IS loses many mechs that were at least performing decently in the current live build (though still notably worse than the best clan mechs). Slower builds may become more popular but they will generally carry less impressive loadouts compared to the faster clans counter-parts.
This is without considering that many mechs in addition to this also
lost weapons quirks for absolutely no good reason at all!
All things considered, I can't in any way see that this is not a relative buff to Night Gyrs, Kodiaks, HBK-IIcs, MAD-IICs, etc. NGRs because their engine is near the base line, and all clan battlemechs because they didn't lose any quirks and can adjust to the agility nerf by dropping down in engine and use these savings for other things, just like IS battlemechs can.
The only clan top performer that is being taken down a notch from my perspective would be the TBR, which has a locked large engine. That's a nerf for sure. The KDK-3 as well to some degree, replacing the big engine with something equally useful may not be that easy.
TLDR: The removal of agility quirks is fine for the IS mechs that are using STD engines, but it can quickly turn now slow twisting XL-builds into death traps. In other words, IS are back to paying the huge engine tax of 9.5 tons for a 300 engine. That's huge.
TLDR2: Hands off the weapons quirks. There is no reason to touch them, the skill tree does not justify any change to weapon characteristics because unquirked mechs get access to the same skills as quirked mechs (both clans and IS ones).