Avgas, I think, has a lower calorific value than Optimax (or unleaded) - but it runs a bit cooler. RS250's, particularly early ones, run far better on Avgas as they were designed to run on it, although most have been converted to run on unleaded (Optimax is best).

You run higher compression on Avgas which gives more midrange torque, but they also seem to get more peak power out as well, but this won't matter much for track days. As the man says run it a bit rich and it will be fine. You probably won't give it the stick it would get in a race anyway.

You need quite different settings for Avgas or unleaded - heads, jetting and ignition curves - so I would just stick with whatever set up you have - measure the head chambers if you are unsure - assuming everything is set up correctly.

I don't think running either significantly affects engine life. I always thought carbon build up was more due to the premix used - 747 is what most people recommend, but fuel may make a difference.

The biggest risk is cold seizures caused by not warming the bike up properly or not applying enough tape to the radiator.

The only problem I had with carbon build up was sticking rings, assuming you clean everything when you rebuild it.

Mike