Just a quick bit of info AJ.
I run an ESP G320 on my X2 along with a GPM 22t pinion and I also run a TTO and GPS system.
Now my X2 on the 22t pinion tops out at 40-42mph with an RPM of around 16,500-16,800rpm.
The G320 still has plenty of bottom end and I have been looking at a 23t myself but I also think to myself that 40-42mph is plenty fast enough for a truck that weighs the best part of 19Kg!
I think that unless you want 45mph+ a 22t pinion will be fine for you, also exhaust plays a big part when it comes to top end RPM on these engines, I've tried quite a few on my G320 and know how they each perform in terms of top end RPM as I could monitor it with my TTO.
I'm still waiting to test out my new BZM Micro pipe but I'm hoping it'll get me to that 45mph mark while keeping the current 22t pinion.