Good question...Must say I haven't heard of this "mushrooming" problem. Can you describe it? Is it a physical deforming of the unibody? I have the strut tower brace and have had no problems (and we race our Minis on high-grip tarmac, over curbs, etc.)...biggest problem is with the windscreen cracking, which the brace helped to fix. So did the JCW cage.
I
can tell you that the strut brace really makes a nice difference in the turn-in. The suspension takes a "set" just that little bit quicker and is more positive. Add a bigger and adjustable rear anti-roll bar and the Mini's handling is transformed....