catmando - look at Kappa that is the boat that Stefano got killed in, twin 900 hp, old style very heavy boat ran 130's if not more (I only remember when she had the small 600's in her and ran well into the 110's).
Gancia in 1990 with the 4-900's over 20,000 lbs, we ran 114-116 mph on Lake Como, that's fresh water, up in the mountains and the boat itself not a very state of the art by today's standards. That boat ran out of boat not power.
You can't get twice the milage with same HP diesel, you get about 50% more Hp per pound between a great diesel and good gas engine. What your saying is why people get disappointed in diesel, they don't understand that diesels are not god's answer to ecomony. Hp is Hp and it takes fuel no matter what the engine is, there is only so many btu's in a lb of fuel.
The problem with parts is there are very few aftermarket parts, most are bolt on and nothing more. You need to make the internal parts you need.
When you think about what you said it would take to do a performance Cummins diesel, why wouldn't just buy the cheaper gas engine??? Everything you said means I can't get service locally, have no parts book, no manual for someone else to work on it, and so on. What happens when Pat's transmissions break, I can't pull into Cummins Mid Atlanitic or Mercury to have it fixed.
The problems are that by doing what you and I suggest (I'm on your side) we're no better than a gas boat except we spent allot more money and have something that no ones wants to ever buy when we're done.
Look at Nortech, they have the ability to build hot rod diesels but they chose to put stock Yanmar's in, with not to complicated drives because despite the fact Jass or Bob don't go as fast as they would like the boats run forever and ever and when Bob goes island hopping (I'm so f'in jealous!) should he break down there are Yanmar parts available all over the globe and people within a couples hours to work on them.
Building hot rod diesels to spank a gas boat is nothing more than ego not good business sense.