The material doesn't come off of the friction surface when a flywheel is lightened, it comes off of the back. Why would you think that the friction surface is lightened?

If you're going to get an aftermarket flywheel, I usually lean toward chromoly ones. I still don't trust the aluminum ones with a steel friction face, it's just too hard to pass heat from one material to another across an interface like that.. There has been a lot of improvement in that area, but only with some of the companies.
The chromoly ones are really nice usually, like the ACT or Toda parts..