MM claims that strut tower braces are mandatory, esp. for track use.
you'll be fine with out it, but i suggest getting one if you plan on pushing the car... ford didnt just throw $100.00 worth of metal away on each mustang made from 1994-1998 to "look pretty." that chunk of change would have quickly added up for something useless.
mine rubs on my CAI as well... i feel ur pain :-(