I have been running my Castle 5700 on this car for a while now - no issues. The track I run @ has a pretty good back straight and fun infield, and the JL + 5700 combination works pretty well. The only thing I added was a heatsink for the motor. After a hard run with a 5000 mah 2S pack, the temps are still within range (even lower with a 20-21T pinion) with the stock spur gear.
Jut make sure your motor mount and the top mount screws (to adjust pinion mesh) stay tight. My screws back out a bit after a few runs, even with loc-tite. When I first bought the kit - I purchased extra bevel diff gears, outdrives, and diff plates. Only the diff plates needed replacement (so far)