i was using my old bag of Excel .25's. Gained about 17 meters from a 20 meter target clearance. Power was boosted from a stock G17 (no upgrades) 275Fps to top out around 306fps on those .25's. I fitted the TK Twist on it gained a few increments on FPS.
Accuracy wise, the thing had a slight improvement and made my groupings tighter maybe around 4-6cm closer to each other from 20 meters with the stock inner barrel. The TK Twist didn't really make a difference based on my observations. Although, others may have a different experience altogether.
and yes it will work with 20s. Tried it but accuracy on .20s is poor at 25 meters. So I switch to 25's.
I think they did tests with .20s on youtube if want a better example.