The process you do currently makes the paint hold up very well... I haven't had issues as far as paint peeling or flaking off...
Of course anything that constantly rubs the inside of a body will rub the paint off in that spot, that's a given...so that happens (of course)

The only thing I could think of to add a nice finishing touch is to add a strip of metal tape on bodies that bulge out on the side to accomodate exhaust pipes.
Truggies and GT's mainly. This (again) is not your fault because it's a case of heat/rubbing going on but it would be a nice touch.

Only thing is I have no idea if there are any ill effects to adding metal tape like that. For instance someone tries to peel it off and it takes all the paint off with it.