Worn rings rarely affect top end bhp much, if at all.
The gas hasn't got time to leak past the worn rings or bore before the exhaust opens anyway.
It's oil consumption or smoking on the overrun that gives it away, but as you've found, it can also be guides or, more likely, just stem seals.
Piston rings.
Re: Piston rings.
It's not falling off, it's an upgrade opportunity.