caltexflanc
Guru
A whole bunch of ifs....
Investigate them all...as there is no right answer generically, but specifically to your boat, maybe running on one could work out...
Can't say your mechanic is right or wrong without those specifics.
His speed range advice might mean he has thought it all through and is right on in your case...or he is one of many in the marine field that once he heard something and tries to apply it across the board.
His emphasis on SOG, among other things, indicates any semblance of intelligent thought has had little to do with his opinion.