Thanks for great info. Looking at those names but also a Helmsman 38E or 43 Pilothouse, and they seem to offer quite a range in terms of engines, so that sparked my question.
Ok. The full displacement hulls won't benefit a bit from the higher hp engines you asked about. So your question only relates to the Helmsman and similar semi-displacement hulls.
First the semi-displacement hull is not as efficient as the full displacement hull at displacement speeds. So just to go as fast as a full displacement hull takes more hp, sometimes 50% more.
The Helmsman 43 is listed at 35,000 lbs dry but could easily weigh 45,000 lbs with full fuel, water, crew and gear. A rule of thumb (confirmed from data in Bebe's book) is that it takes 1.5 hp per thousand pounds of displacement to push a full displacement hull to its displacement speed. Displacement speed is 1.34* sqrt(lwl). So it could take 68 hp to do that and over 100 hp for a semi-displacement hull. That really means a 200 hp engine so you don't have to run it at red line all day and also have some headroom for pushing into head seas.
So, Helmsman offers the Cummins QSB 6.7 250 hp engine for that boat. It would be a good choice but you can't expect to cruise at more than displacement speed with it. But they also offer the 480 hp version of THAT SAME ENGINE.
There is no mechanical difference between a 250 hp QSB 6.7 and a 480 hp one, just computer chip tuning differences. You can buy a 250 hp version and later if you think you want more hp, Cummins for a price will change the chip to let it produce 480 hp. It will probably take a prop change or maybe a prop and transmission change, but the base engine stays the same.
I am guessing 480 hp won't do you much good. It takes about 100 hp to hit displacement speed or about 8.5 kts. With 300 hp which is probably all you should cruise the QSB at, you won't go that much faster, maybe 12-13 kts. A good comparison would be the Mainship 400 with the Yanmar 370 hp engine. Mainship's old website says that boat and engine will do 13 kts at 3,000 rpm which roughly equivalent to the QSB 6.7 at 300 hp.
So does that answer your original question? In summary the higher hp engine isn't any different mechanically from the 250 hp engine, so fuel economy and longevity will be the same at or below displacement speeds. It takes a chip and maybe a transmission and certainly a prop to make and use 480 hp and that will maybe get you another 4 kts.
David