The big changes are only really relevant if you run in a stock class (13.5t or higher wind.) The new generation of ESC's have advanced timing and dynamic advanced timing (you can change the motor timing as a function of RPM). This gives a higher top speed without increasing the temps too much and without impacting on the low speed acceleration.
If you are running mod, or any unrestricted class then I'd say stick with your GTB and if it's not fast enough then you can run a faster motor
Ben