Does it still have mouse accerelation after your fixing?
Lowered resolution and vsync on -> no acceleration. Now I have to try another mechanism to limit framerate without vsync. I already tried a d3d9 wrapper but that also gave lots of microstuttering. I never tested it at lower resolutions, however.
EDIT: Tested wrapper again. It can do a decent job of maintaining framerate without stutter but for some reason I'm seeing significant mouse acceleration. What the hell? Also, sensitivity with the wrapper is quite different from sensitivity with vsync, even though both maintain the same framerate. This kinda suggests that there's a separate input thread that somehow is affected by vsync but not by a sleep in RenderPresent()? Go figure.
EDIT: This is the wrapper I tried: https://github.com/ThirteenAG/d3d9-wrapper/blob/master/source/d3d9.cpp
EDIT: Still getting microstutter and jank that affects mouse response. Combined with vsync latency, the experience is just... nngh, very irritating. I can't feel immersed at all when I'm constantly up against this nagging mouse jank. I switched mouse to 125Hz poll and that does not fix it either. What the hell.
EDIT: Aand there we go, bullet sponges that don't die to three shotgun blasts in the face at point blank range. Yeah I'm done. Dang, I had really high expectations for this game, but it really manages to pull of just about every mistake (gameplay & technical) one can do with a first person shooter...