the "soft" side should be higher grip actually. i haven't run on RCP since the first time i touched a dNaNo but i believe its surface is similar to ours so i would recommend either 45F/10R or 35F/8R (this means you'll need a combination of PN Racing, Atomic & Kyosho tires depending on the combination you choose).
the porsche 911 gt3 is prone to rolling because its very narrow. high front traction when cornering at high speed will almost certainly cause this problem. i reduce the steering angle (widens the turning circle) to lessen the problem but throttle control & braking before entering corners will eliminate the problem (reduce your breaking strength to just above minimum).
if you don't have a ball diff & aluminum wheel hubs, get those to add weight to the rear and increase rear traction as well. last but not least, pick up the atomic t-plate set and find the right rear suspension setup for the RCP surface. have fun and post some pics (here & on our forum) when you have them
