You also said capping fps didn't work when vsync is on, and it wont, thus turning off vsync. If you have a 60Hz monitor cap maxfps at 59/60/61 play around see what's better and although you may get a little tearing with no vsync buffering it wont be as bad as not capping fps with vsync off.
Im using a 120Hz monitor have vsync off and fps capped at 121 and get no noticeable tearing at all.
So whats worse mouse lag or a little tearing ?
I do believe the mouse lag with vsync is worse on linux than windows. lets hope fixes are on the way