The sleeve mod as shown in the middle pic of the first post is an easy one to do, and it works...
Also, a guide channel is even better. If you have seen the sleeves out of some of the high perf. R/C car engines, you know what I mean.
I have done it on many engines, and have seen at least a 300 r.p.m. increase and as much as 500 with all else being the same.

I did an O.S. .61 FX because I thought it needed some help, and saw almost 500 r.p.m. after just working the outside of the sleeve.

Also, I agree with Dave, in that you don't want to break the mechanical bond of the chrome inside the sleeve, or you're just asking for trouble (peeling)...

The best Dremel cutters for that job are the flame shaped carbide, and the 1/8 and 3/32 dia. cylinder shape.
If you can hold your breath for long periods, that helps too (helps keep a steady hand)