I have a 750 motor and I am running 50:1 at 150 PSI with no problems as of yet. I switched from 40:1 to 50:1 because my blaster friends and my dirtbike were running 50:1 with no issues. Also, my motor likes to smoke a lot. I'm thinking it smokes a lot because the exhaust is not tuned and the temps aren't high enough to burn off the smoke?? I don't know, but going leaner on the oil reduced the smoke as well as my costs. However, I switched to premium unleaded at the same time so I'm actually spending more $, heh.
The person I bought my 650sx from was running 40:1. I ran that until water ingestion killed it.
40:1 or 50:1 doesn't seem to change anything (naturally I don't switch mixes after tuning the motor).