Last on threw me, until I saw the heading on it, I was going to ask if you used a different car.
It's all about getting out of the hole and theire are verious was to do that, I'm not very good and it takes a lot of practice. The best time I have seen from a GXP was Rusty at Cecil county and he turned a 13.85, but he does work there and I think knows all the tricks.
Joe's best time that same day was a 14.01, mine with a stage I supercharger was 14.102.
Tire pressure around 20 lbs, you should be able to launch around 2-2.5k. Some slip the clutch others dump it. Not sure which way is best, with my stage III, I have been launching at an idle using both clutch technics and just burn the tires off in 1st and 2nd.