Josh, I ran into a similar problem with corrosion on the plastic water intake fitting that comes out of the outdrive. Your inlet hose to your oil cooler or sea strainers is hooked up here.
When I took it apart the inner diameter was the size of a pencil, boat would run all day cool at 3500 rpm, but as soon as I increased rpm,s the temp would rise. Bring rpms down and it would run cool!
Good luck , post what you finally find the problem is!
Brian

A bunch of rubber from the previous impeller caught in the oil cooler tubes, pulled off a few hoses and flushed it.

Seemed to stay pretty consistent at 175 degrees. Inched towards 180 a bit when I ran it hard. To me the temp still seems high, but everyone else says thats a normal temp.