yes in some engines this will help in a noticable way and in some engine's it wont help as much. You can also in some engines the exaust port in the sleeve is a little bit smaller then the exaust port is in the crank case. with the sleeve in the crank case you can take a marker and mark the excess metal on the sleeve and with a steady hand dremel it out. But i wouldnt do this if you arent good with a dreamel and if you dont have a steady hand as one slip up can cost you a lot of money.

But yes this is true