Sounds like a jetting problem to me. The change in oil mixture should not affect your powerband too much. But if you're experiencing "bogging" at high rpms, you may want to adjust your main jet.
In my RM125, I've used mixtures all the way from 80:1 down to 32:1, and the only thing I've noticed is how much splooge I get coming out of the exhuast pipe. I've also noticed that if you're not on the pipe (being in the high rpms) all the time, if you do decide to get on it, it's going to bog a bit which, I believe, is due to the fact that there is unburned oil built up inside and it takes a couple twists of the throttle to get it out. After that, it's gravy.
If you need help jetting, shoot me a PM. I just redid mine not too long ago.
Hope this helps.
Brian.