What's wrong when there isn't much change from 1/2 throttle to full throttle ? Twin 450 hp 5000 rpm 62 mph . Thanks

My boats the same way man. Once I'm on plan and moving the last 1/2-/14 throttle is about 6-400 rpm.

Carb or efi?

Carb, it acts like I wouldnt need a 4bbl. The carbs are off a hp 500

most likely jetted to rich.

Or props to tall

+1 and or too much carb..

If to rich do you just lean that side of the carb.? (Full throttle)

Run it wide open with a fresh set of plugs and check your plugs. If your so rich that you get nothing past half throttle it should be relatively apparent. It would be good to know what intake vacuum is at 1/2 throttle and also what power valves you have, that way you could tell if the power valves are opening at that point and causing you to run rich. To lean out the carb you need to change the main jets which effect everything past idle.

Power required effectively goes up with the cube of the velocity, so you aren't going to see a tremendous increase once near the top end of the power spectrum.