I have a 200 HPDI on the back of a 23SC Parker. They seem to run fine but use a lot of oil on the break in period. You also should use a fuel additive called ring free and it is not cheap. The engine itself runs quiet at an idle but when running higher RPM'S it is about the same as the saltwater series. Overall it is a sound engine, never have to use the primer bulb except om the begining of the season.
I just came back from the keys and had an oppertunity to go on a 31' Intrepid with twin 225 Yamaha four stroke. I stood by the transom and the capt. was by the helm and we had a conversation while he was running about 3800RPM. It was really quiet. I don't know about the ring free being used in the 4 stroke.
The nice thing about the HPDI is that it meets the 2006 EPA standards for emmisions if you are into that. The HPDI is also great for trolling because of the lower idle.
One thing though, make sure you stay within the guidlines for the break in period. They can hook up a laptop to the engine and see how you have run the engine. It breaks it down into different catergories of RPM'S and if you are not in there parameters you can lose the warranty on your engine. The break in period is ten hours which is nice but beware of those RPM'S. When you do the 10 hour check make sure they adjust the oil consumption, it it usally set high for the break in period.