GG is the medium compound for R888s - you could switch to the SG compound which is soft. Even better switch to Kumho V70A in soft compound.
It does sound like you have too hard rear suspension. Most of the L7 competitors are running somewhere in the 135lbft to 150 lbft range. I am using 150lbft, relatively stiff damping, Kumho V70 in soft 235/13 and get consistent 2.2s times.
The other variable is gearing. One of our competitors has a very tall 1st gear and the best he can get is a high 2.4s launch.
Yellow SL #32 - member of Drowned Rat Racing