I haven't completely disassembled the engine, but I have taken the backplate off, and there doesn't seem to be any play in the moving parts, and the sleeve is still polished-smooth with no scratches.
If it were just premature wear, wouldn't it have trouble idling all the time? It seems to idle just fine when it's not screaming-hot; in fact, when it first starts, it idles so high the clutch drags a little; it settles down after running for about 30 seconds, and idles smoothly, until I put it through a few speed runs, at which point the idle drops so low it stalls.
So if heat is a factor, what can I do to mitigate heat-related misbehavior? Would reducing the compression ratio with a thicker shim help at all, or am I pretty much looking at getting the sleeve re-pinched as my only viable option at this point?