I tried raising the sleeve by 0.2mm and saw no beneficial effect after re-tuning the carburetor. It ran hotter, didn't idle as well, and had a slightly lower top-speed. (yes, I remembered to remove shims above the sleeve to compensate for adding shims under the sleeve, to maintain the same compression ratio.) So I guess that's the end of it -- a 0.3mm shim-stack with no raising of the sleeve is the best configuration for this engine. If I used a cold glowplug, maybe I could run a 0.2mm shim-stack, or a 0.1mm shim-stack with the sleeve raised 0.1mm, and maybe there would be a benefit, or maybe there wouldn't be. I'm not sure it's worth further experimentation at this point.