I'll just throw something else out there to cover the base. You mentioned you removed and reinstalled the piston rings to inspect and check the gaps. Are you sure you didn't mix up the 1st and 2nd compression rings when you reinstalled (and did the original builder get them right)? And reinstall in proper orientation if they were marked for top side? Just figured I'd ask. : )
The rings was something I researched hard to make sure they went back in the right orientation. And proper gaps.. the only thing I can maybe think with the rings is the bores aren't perfectly round causing some compression loss.