Actually, Director does support 60FPS too and I did play Metal Mayhem World Tour in 60 FPS and it is really smooth. I do know if you mod BR3 to run in 60fps, it messes up the timing. This is also a problem with BR4, but both games support 60FPS fine. Maybe you can fix the timing issue so enemies don't get to shoot faster and you slower.
However, I actually like 60FPS for BR4 as it makes it a bit harder and everything is smoother. If Xform has time, a proper 60FPS would be nice, but look, they just released an awesome update which fixes a lot of the real problems. 60 FPS is a minor thing.