I've managed to get higher rpm out of my Sachs but with loss of power at the bottom end. I have also cut donut spring a little shorter, not much but it is noticeably stiffer and it still engages at around 3000rpm. I think the power band starts aroung 4500.
I think the next step is a better exhaust but f'ing expensive getting one sent to OZ. Been trying to find one that I can Retro fit.
I suppose I can always change the gearing, maybe one tooth more on the back sprocket, or one less on the drive end but that would loose top end.