If the lifters spall, become rough, and wear down the cam, there will be
almost no roughness. The engine will simply lose power as you describe. It
may make static RPM but not reach full RPM in cruise (it's a bit more
complex with a CS prop).

Oil analysis will not show the big pieces of metal that come off the lifters
and may miss the cam shaft wear as well. Have the filters been cut open and
carefully inspected at each oil change? It takes more than just a look.
They need to be drained and inspected very closely. Washing in solvent and
running a magnet around in the pail is recommended by some.

This is just one possibility but it's very much on my mind since our engine
is currently spread out on a rebuild bench.

--
Roger Long