I'm going to chuck this one amongst the workshop techs,in the mean time,if you go out again,try running the motor at the necessary rpm but at a different hull speed, so SB motor only as fast as necessary to get port rpm to 5300. I once had a through hull pick up causing just the right amount of water aeration round the water pick to cause an overheat, either side of that speed was fine. Weirder things can happen that one doesn't think of. I have never had a BF150 behave this way, usually the overheat occurs above a given rpm.So if it doesn't overheat at a lower boat speed you need to look at the hull to see what could be causing it.