To my knowledge the rear arches are structurally important, on the sapphire any way, not sure if it applies to the 3 door. If you start cutting, folding or grinding them back it could effect the torsional rigidity (I used big words) and knacker the handling.

On the other hand, I could be talking complete crap, if so please ignore the above and enjoy the following tune,

tumm tee tee tumm, tee tee tumm, tee tee tumm, laa, la la laaa la la tumm, tee tee tum.