Well, it seems I had two issues - neither of which was the impeller.
The first was some scoring on the inside of the pump cover panel. I ground it smooth, and that doubled my water
output from a trickle to a sprinkle.
I had put all new 3/4" hoses on the system. What I wasn't realizing is that while the pump had 3/4" outlets, all the fixtures on the thru-hull and raw water
strainer were 5/8". Since there's not really any pressure on those hoses, they weren't leaking, but they weren't air tight. As soon as I switched to 5/8" hoses, it started pumping water
out the exhaust
with no problem.
It's crazy what one tiny air leak can do.
Thanks for all the help.