very common occurence with the 911... you need PN Racing 45 degree tires (Kyosho 40 gives u too much grip while 50 is too hard for RCP/Rubber; stock is F/20, R/10 btw). you can make steering angle and servo reaction adjustments to counter-act this as well. if you know what you're doing, the 40s will fine. my setup is F45/R15 and its tough to beat the times it registers. i've even done well with the Atomic 35's but u will need to have patience and pretty good with throttle control to keep it on all fours.
u should run a 2-plate setup in your ball diff (with or without the containing spacer is up to you, i go without). if you're using the Perfex KT-18 Transmitter, back down your braking to a minimum so you don't lock up and cause the roll. those tips should get ya where you want to be but due to the fact the 911 GT3 is most narrow track width, its automatically prone to this problem.
good car choice though
