A propeller will spin fast with more hp....unless the load against the motor will not let it do so, ie: the boat itself just does not want to move any faster.

Impeller's are totally different. They are more like a torque converter or better yet, a water brake on a dyno.

That's all I know. Absolutely no jet experience what so ever other than reading basics on them.

well its been a long time and i have an update. i found out during my trip that my impeller was F'd. in swift water it was cavitating. thats not the main issue though. so i had no clue where to begin to solve my cavitation problem. was in the impeller, stator, other. i had no idea where to start.
the impeller was just worked on so it should have been good. i had that low RPM at WOT issue because the motor wasnt making enough power etc. so i was able to get 2 different impellers to test yesterday. i found one 2 sizes down and the same one i have. the downsized impeller allowed my boat to turn 4800-4900. i havent looked at the datalogs to get exact. in the whitewater once it unloaded i was screwed. it didnt have enough push to get my boat going in the right direction once unloaded. i also got to try the same pitched one as i currently have. i just did a speed run to see what RPM it would run. it ran exactly the same as my F'd up one. you could feel the difference in push though. so it appears i need the one in between these two.

If you actually knew where the cam was placed in the motor, you might try advancing it another 4 degrees before changing it. By doing so you will lower the power range by several hundred rpm without losing your peak HP.

I'd try running a "C" impeller. Jets like big block torque.

i cant remember where the cam was installed. i think it was installed exactly as need be which was advanced.
its a hamilton pump, not a berkley, american turbine or scott.

If you didn't use a degree wheel it's impossible to tell. Lining up timing marks doesn't mean it's right.

On my motorcycle engines I've found the cam timing marks to be off by as much as 4 degrees.

my builder put it in. not me.

talk to Duane at hi-tech performance he is the best in the country when it comes to pumps. I've worked with him for years. He can steer you properly