I am curious on this topic. I recall the first time I attended a race where the altitude was very high. I couldn't figure out why my engine was running so hot and had no power. Nobody could. This was back in the early days when I was still pretty new. I look back on that event and my buddies and I still discuss this.

I recall the solution was as already mentioned, alter the head clearance, or even run more nitro. Like 40%??

Ideally in high altitude what should be done??