I looked into this when I had one, because theres a lot of rally/competion cars that run 13" wheels or less eg. Mk 1 Escorts, Minis etc, and I've seen some slim AP racing calipers for these. I don't know if its true but could increasing the size of the master cylinder help? I saw this mod on Australian Ebay for a pulsar which ran 500bhp, but was totally stock to the untrained eye inc std wheels, but competed ( and won) events against V8 muscle cars and the like.