As I mentioned I did do a leak down and found a leaky exhaust valve and got that almost cured - Under relatively low leak down pressure (30ish psi) at TDC there was not much blow-by as evidenced at the case vent - A bit more on the rest of the stroke but I suspect dynamic compression is OK. And that's why it runs good. This was the side I did make a ring for (which helped a bit - old one had excessive gap) but I suspect a good set of rings would help even things out - The bore looks pristine but haven't checked for out of round.
Cylinder leakage is a odd dynamic - In my past have done 100's of leak downs on TIO-540's in a legacy fleet of Piper Navaho's and Chieftains and it was always a mystery why some would show signs of power issues with "limited" sealing issues and other could have terrible cylinders and still run fine.. we tended to lose cylinders at about 1500 hours (on an 1800 hour TBO) especially on the 350HP version. Probably had to to with the turbo and control system with some being able (and set) to just "jam more in" on some engines. I miss running Turbo'ed Flat sixes. It always made me smile and gave me goosebumps when the Boost would start rolling in - they make quite a beautiful sound!!!