Okay, so raising the sleeve is worth trying. Do you have a recommendation for raising the sleeve 0.1mm or 0.2mm? I assume if the sleeve is raised too much, the engine performance starts to decrease again.

Regarding your other points:

I checked for interference between the piston and cylinder head. There was no evidence of impact with a 0.3mm shim-stack; I haven't opened the engine to check the result of the 0.2mm shim-stack, but at least there was no interference when the engine was cold, and I didn't hear any unusual sounds from the engine when it was running.

Why would advancing the ignition timing lower the max RPM? I thought high-RPM engines need advanced ignition timing to ensure the combustion completes before the piston starts to descend. Every time I've tried to get more power from an old engine, a hot glowplug improved the performance, and then I knew I could increase the compression to get the same effect with a medium glowplug. I've ever actually used a cold glowplug in any of my engines. Keep in mind, my fastest engine has about a 38000rpm max, which is still about 10,000rpm lower than race-grade engines.