I would be tempted to up pinion, the 5200kv i run is on 19t, great top end if you need it for long straights and it hasn't really lost the any bottom end, the 4 pole should definitely handle it, i have ordered a similar motor to use in the longer chassis, looks actually identical to yours, i linked to it on banggood up a few post's, i will most likely try go 20t pinion on it when i get it built up that is.
The gtb's are good i have one too but favour the emb, the mini 8 sits there next to it lol, gtb is so much lighter even with an alloy chassis i have put on and geared diffs over ball not that that adds alot.
Si