Hmmm. I'm betting there's something else going on - maybe the impeller is weak... maybe the housing is scored... maybe there's an issue with the muffs? My mother has a 150 and it works just fine. I guess if you think about it, there really shouldn't be any difference in the impeller (for the most part) from one engine to another. That part is pretty standard.
That would bug me until I figured out (fixed?) the root cause
But hey, it is what is! And, in the end, you built a better mousetrap
EDIT: Just wanted to add... sometimes when we're using a big jug of AF to winterize a sterndrive, if we open the valve to let the AF flow BEFORE we start the engine, we get an air lock and the AF doesn't get sucked up. But if start the engine and then immediately open the valve, it works everytime. Maybe there's an issue with the muffs?