Jeff, I amuse myself most of all!!! (I'm not a naval engineer or prop master, I guess at speeds based on experience and test data regarding how much of our useable Hp we could apply to the water, we built engines and engine parts, boats were just another platform like a dyno to test engines on)
I can tell you this, a true 360hp being used on a 40' SH loaded with lead so it weighed 15-16,000 lbs was 65 mph.
The 38' Ebel with 600hp runs 92-94 mph (we would hit the governor not run out of Hp. Put larger props on and you could not get on plane - remember single speed trans then)
Todays hulls are faster, lighter in general steps etc., props are so much better, I would guess your 480's have to put you into the low 80's?
You need 600hp to run how fast you want to cruise. My point is that even with that extra Hp without the taller gears you won't go fast enough because the prop speed limits you.
I have a question for you, whatever boat your talking about how fast will it go with 500hp gas engines?
ps - in reference to the Ebel 38' (SH#38), when we changed engines to big twin turbos 850-900 hp each we did not change gears just props you would expect the boat to go like hell now??? it ran 102 mph but accelerated so hard it would push you back in the bolster. Again, the limits were gears. When the Gancia was repowered with big engines it had much taller gears than we had on the Ebel, the Gancia in freshwater (Lake Como), high alt ran 114-116 mph and had plenty Hp left, the boat could not run faster.