You need traction to run a 6T. 7T's can be hard to control if you aren't good on the trigger. With a 6 or 7 you can light up all four tires out of any corner. 8's aren't as bad.
The GP3700 are the choice of the top pros. The early IB3800 had good numbers but were pretty poor on the track. IB seems to be improving all the time so if they aren't good already they will be soon. The GP's are still the safest bet though.
I just got some Fusion GP3700's 447-462sec@30A, 2.0-2.2IR, 1.180-1.189.
Thats good enough to have ~50sec left after a good run with a 7T.
What I run: Schumacher Mi5/Associated RC10R5.1/Associated RC12R5.2/Futaba/HobbyWing/Team EA Motorsports/BSR Racing
Where I run: Florida Indoor R/C Complex/Thunder Racing/Florida On Road State Series