You won't know the answer to any of those questions until you have a chance to run WOT with the 21.
After that run, you can use the results to make a prop decision. No doubt in my mind that the 17 is way under propped for your rig. Holeshot will indeed be stunning, that's where the benefits end. If you leave the 17 on that boat you will burn more fuel than necessary, create more wear & tear than necessary, run the risk of over-revving when you have an opportunity to get on it (or more likely just slam right into the rev limiter), which will give you very limited top speed, etc...
For example, you say you need to spin 4200 rpm to go only 40 mph now. That's insane for a 300 hp V8 on a 20 ft runabout. Your motor should be more than capable of cruising that rig at 40 mph at only about 3000 to 3200 rpm. Currently you are doing the equivalent of taking your every-day driver vehicle out on the interstate, putting its 5 speed automatic transmission in 3rd gear manually (or maybe even 2nd gear), and then setting the cruise control at 75 mph and just letting the motor spin like that for miles and miles. Your throttle response would be fantastic, while everything else would suffer. I think you'd agree it would be rather senseless to do such a thing with your car on the highway...
And it makes no more sense to do it with your boat.
Run the 21 and report back. That 17 needs to be sold. It will serve as a fine spare to get you home in an emergency, but otherwise it has absolutely no place on your boat...sell it to someone with a 3.0L...many of them run 17's.
Like I said earlier, you have a 60 mph boat...EASILY. Many people on this board would kill for a boat that nice, and with so much extra power to spare. Now that you have it running properly, don't waste its fantastic potential by propping it like a 20 year old 3.0L runabout LOL...
Enjoy it!
