I have modified the sleeves of several engines as Asmund described...

I typically see about 4-500 r.p.m. increase with all else being the same as before the mod.

Example: O.S. .61FX was 11,100 before, and 11,500 after. 10% powermaster, APC 13X6 sport, slimline muffler and #8 plug.
(it was in a 9 lb. Corsair, hence the big prop )