Shaft Vs Belt
The concensous seems to be.
Shaft lower rolling resistance = more efficient = quicker therefore better for Stock and 19T
But - Shafts introduce gyroscopic effects that apparently have detrimental properties under acceleration and can effect ultimate grip, hence with the very low turn Mods now in use it is said that Belt drives generate more grip and the that the loss in efficiency is negated by the extra power of the motoer.