As I read it, engines such as io550d's will be eligible provided they meet the usage criteria, ie average 40 hours/month. This allows a 200 hour tbo increase.
I don't agree that going over tbo by certain amount, will increase overhaul costs. That hasn't been my experience with 520 and 550 conty's.
By that, I mean 200-300 hours over..
They suffer more issues from lack of use rather than high use.
Cheers.