The other thing to maybe look at are the valve seats/faces and/or guides themselves...?
Everything is pretty small on these things, but it might be worth giving the valve seats and faces a decent eyeball - make sure there isn't a ding somewhere due to ingesting a bit of junk, or a scratch or burn-out excessive wear... or just bad engineering somewhere (??)
BJ
PS - I'm guessing there's no cracked rings... holes in piston crowns or anything else obvious...?