[willy]

Did the so called commercial engines have a restrictor on them preventing RPM advance or retarding advance. I know some commercial equipment you can rent have those on them. Just a thought?
Also can you rev the engine out of gear (in the water) to above that RPM. If it does not go above like that then I think you have a timing issue.
Also what is the condition of the lower unit, if it is under load and won't reach the desired range but will do it with no load the lower unit can be causing the problem with a lube or bearing problem.
In the carbs you can also have a restrictor plate preventing fuel flow above a certain CFM
Just some thoughts, have you ever gotten the manual on those commercial engines, if there was any restrictor type things it should be noted in the shop manual.
The reason I think it might be related to something of that nature is you changed the props pretty substantially and saw no changes in RPM's. I never heard of that ???