I run 17.5 buggy. I currently have a Novak GTB2 (non x-drive version) and a Schurrspeed 17.5 motor. I have a Brushless hobbies LT4 80a esc on the way to replace my sons melted SP citrix in his SCT. Would I see a performance gain by running that LT4 instead of the Novak?

I doubt it. The GTB 2 is a good racing ESC capable of handling up to 3.5T motors. It just doesn't have timing adjust. If you run "blinky" I doubt you will find any ESC gives much improvement over the Novak. If, however, your class allows timing advance you will be at a disadvantage and the LT4 might improve speed on long straights.