B7200 Oil Pressure

I have been chasing down the cause of low oil pressure on my 1986 B7200 HST 4WD.

I've changed the sending unit, pulled the timing cover to inspect the front cam plug (it was intact), replaced the oil pump and oil pressure relief valve, but still have only about 12 psi (at idle) and 24 psi (at 2500 rpm) when cold, but it drops to about 4 psi (at idle) and 10 psi (at 2500 rpm) when warm.

I don't have the know-how to get to the rear cam plug, and the local Kubota service guy says there is only one on the front anyway (although the parts manual shows one on each end). He says an engine overhaul is the only option. The engine runs and sounds fine to me, but he says it's noisy when throttling back down, indicating mechanical wear. Should I insist he check the rear cam plug, since if that's missing an overhaul might not be necessary?

So are worn bearings the only other reason for low oil pressure?
What if I do nothing and run with the marginal oil pressure? What more damage can it do?

Thanks; I had completed all of those except the bearing clearances. My Kubota mechanic says it will cost big bucks to tear down and check all clearances; he doesn't think it is worth it!

The mechanic also thinks he's hearing piston slap, so the cylinders may have to be bored and oversized pistons installed.

What is your opinion of need to rebuild the engine due to the marginal oil pressure?