Well i've ran it a few times now. Was using a 14T pinion to start with, but gone for a 18T pinion now and tested that out today.
I think it ran 40mph ish with the 14T if i remember correctly. Should do more like 50mph on the 18T, but i didnt put the GPS on it yet.
The motor was only hitting 40C on the 14T pinion. Pretty cold really after running flat out on grass. With the 18T pinion it only was about 60C.
Also an update on the tyres. They are great. Zero regrets not waiting for the MX SC to come back in stock.