Just swapped out a Novak edge for a gtb 2 lp xdrive. Both escs fit in the fgx without a problem although the gtb 2 is a little bigger. It has a larger cap too so you need to get that settled in there. Again no fit problems with either.
As far as performance they both run great. They are both very tuneable. They both had the same lap times but the gtb 2 felt smoother. I wasn't any faster but I was more consistent.
3Racing Sakura XI 17.5 TC
Formula Rabbit (FGX conversion) 21.5
STRC LCG 2WD Slash 17.5
Serpent S120 13.5