depending on the offset of your rx7 rims, i'd say the safest widest tyres are really a 185. maybe if you ran spacers on the fronts, and pumped both the front and rear guards you might get 225s under there. as jamie said, probably not the best for wet weather handling, also be very baggy.
with my 13x5.5" mazda rims and 185/60 tyres, there is less than a fingerwidth between the front strut and the sidewall. the outside of the tyre is inline with the top of the guard. on the back, i had to lip the guards on the ke15, the ke16 doesn't scrub, but it has stiffer rear springs.