Howdy,
I'm racing a T4 with the GTB and Velociti 5.5. I've been racing it about 4 months now. I'm pretty happy with it.
The fan blades all snapped off in the first couple of weeks. For a while I ran without a fan at all (no problems) then I got one of the AE fans and glued it to the chassis just near the ESC. I've had no heating problems so far and it all seems to run reasonably cool.
I'm running (from memory) a 17T pin and stock spur. There is a heap of performance, and I think at a lot of tracks I would prefer to have a 6.5 or even 7.5, but our track is big and fast and I can usually manage to get to full throttle for a second or two down the straight and maybe one or two other places on the track.
For racing, the biggest difficulty is the ferocious bottom and mid range. On a brushed motor you can tune this out with brushes, springs and timing. On the brushless it ain't so easy. At the moment I'm running about -70% throttle expo.
All in all, I'm happy with it.
Mike B