i ride a gasser, but i have to say if you watch the world rounds and what you can do on a 125 or even a 200, i can guarantee you're not hitting those caps.
the older 125 lads are around 12 stone and are hitting 6ft plus rocks in their final year
youll notice the gasser maybe smoother but itll also be quicker on the harder bits and get away from you faster due to being more powerful. every bike has pros and cons
youll be surprised what the small bikes can do.
Id put the suspension and carb settings to completely standard maybe a few click off on rebound to make it less bouncy and put hours and hours in.
dont be scared to crash or rev it.
re beta parts, my gassers absolutely butchered with trick bits, certainly doesn't help (hasnt helped me) until you at a level you start snapping frames...
there's riders at british rounds on 250/300 betas with stock suspension, there's no need even on club-level expert sections to have modded bikes unless you have an itch to scratch with it...
Do love gassers tho....