Read through this. Yeah, I know. It's nearly three years old by this point, but it seems to be the universal go-to tweak guide for all Unreal Engine 3 based games, up to and including both XCOM 1 & 2. Using it, and (weirdly enough) switching to an earlier driver rev, I've managed to push the Linux version from nigh unplayable, to perfectly solid, if not absolutely astounding, with everything turned up, saved for texture resolution and draw distance at 1080p. I'd say I'm getting about 30-40 FPS during the base scenes, and about 45-60 in missions.