I can't rememebr the exact figures but under heavy braking about 80% of the weight/effort is done by the front anyway, so you may not need to bother with the rear. Unless of course you want to upgrade them just for efect.
I don't know if a set of GTR front will fit the rear of your car but, bering in mind the number of people who have upgraded the front set there should be a few spares knocking around.