I'll give you the best numbers and info that I have gathered over the last summer and see what advice you can give.
The boat is a 1981 mid to heavy layup 21' Daytona, approximately 3000 pounds dressed, most likely thought out to be setup for outboard and turned into jet.
American Turbine pump
Unknown droop 1 1/2"
1/2" radius keel
Loader 1/8" minus keel
The previous owner had a 427 NA motor, aluminum A and could get the boat to a reported 73 mph.
I outfitted the boat with my engine, a blown 468 turning my pump internals, into the same AT, stainless A at 6200, roughly 760 hp.
The best I could get was a whopping 75 gps'd mph.
I detuned the engine by swapping pulleys down to an approximate 580 hp, at the same time removing the large rudder. I gained 5 mph for a top speed of 80. I had a great headwind, and packed enough air to get the intake out of the water and cavitate. This boat does not have a ride plate or shoe. While I am not trying to squeeze every single MPH out of the boat, I know it can do a lot better than this. It is getting a ride plate and shoe this winter, but should I consider doing a setback at the same time? What are your thoughts on a setback without finding the precise COG? Does a setback dramatically increase the chance of a blowover? In my 80 mph run with a great head gust, the bow had a nice rise, but never felt unsafe before it lifted enough to blow out the pump.
Whenever you add more power and the boat has a weak response ,you take power away and the boat speeds up, it is usually that the pump is not getting enough water. The boat will accelerate better but once the water acheives a speed that will not allow the water to make the turn into the intake a shoe and rideplate assembly has to be installed. We are right now redoing 2 boats at the shop with the same problem. The 21 Daytona is very pron to not load the pump sufficently when it runs flat. That is why when you ran the boat into a head wind the nose came up and the intake saw more water, thus the boat went faster. Also when installing the rideplate and shoe assembly be sure you get some good help in setting the intake because you can create more problems if it is installed incorrectly. The intake is like the foundatition on the house it has to be right. The 21 is also a good boat for a setback so that should also be in your future. One last thing would be to install a pump pressure gauge this will aid in getting the shoe depth and overall setup correct.