I have gone to HS225MG's on all ailerons now, and HS85MG's on elevators.
All of my original plastic-geared servos have been converted to metal-gear by now too, except one that I am using on a throttle. The only reason that I have not converted it yet is that I still have a few plastic gear sets left for it.

Yes, if you have a plastic geared Hitec, the corresponding MG set will replace the PG when you strip one out.
As a matter of fact, just before the Duel Over The Desert my LHS (Hobby Barn, hobbybarn.com) got tapped out of the MG servos that I use, so I picked up one el-cheapo Cirrus servo from them for a throttle... hahaha, it has survived one mid-air and one serious lawn dart from about 80' that destroyed a gutter-pipe SPAD.