There were a few posts on this subject a while back. If they weren't lost in the crash, you can find them by doing a search.
Personally, I wouldn't remove the bumpers supports unless your car was purely a racecar (i.e. full cage). If it's a daily driver, I could only see bad things coming from removing a safety component.
My only pointer is that you probably want to look towards removing weight from the front end of the car first considering the Mustang's weight distribution.