I don't know if it's a piston clearance issue. I always let my engine builders deal with that. I never even bothered to ask. I awlays wanted to get out of there fast so I'd avoid the lectures on treating their engines better.

When I ran karts we went pretty heavy on the oil When trying to run close to the edge, I wouldn't have considered running 100:1 or even 50:1 for that matter. I don't remember the exact ratio I was running, but it was probably a lot closer to 20:1.

Timing could be another reason the manufacturer is getting 1000 degrees and you can only get 850 degrees. Timing that is overly advanced will cause detonation. If the detonation is not too bad it might not be very audible. Extended running at high RPM with mild detonation can cause migration of aluminum from the top of the piston to the cylinder walls. This can end up looking a lot like a traditional stuck piston due to a lean condition, as opposed to the more usual symptoms of bad detonation like holes in piston. You could try retarding your timing and see if that helps.