the proportioning valve is easy enough to switch...and i have one from a del sol si (has rear disc brakes, and has the same size brakes all the way around) as my car now that i installed civic si rear disc brakes. i've been told that my proportioning valve will cause my rear brakes to lock up prematurely, and this simply isn't the case, as i've made some extremely hard stops from 130+ down to like 25mph and i didn't have any problem with this. for your situation, i'd use a bigger master cylinder, mainly because you're forcing the system you have to stop bigger brakes, which does require more fluid in the lines. now i'm not saying you have to, but it would probably be the right thing to do, mainly because honda gave the gsr a bigger master cylinder from the factory, and i always like to stay as close to factory specs as possible. (remember with my car i have a dx, which uses the same size master cylinder and the same size brakes compared to the del sol si.)
as far as e-brake cables, i'm not positive a gsr cable will work. i know that the new civic si e-brake cable is like 2 inches too long and i am going to install a del sol si ebrake cable. to verify this measure the length of the gsr ebrake cable and go from there, although its my feeling the gsr cable will be too long simply because the gsr has a longer wheelbase.