BTW @SeaNile I saw your post over on THT, and you are getting some misinformed advice over there. People like to act like they are experts because they read someone else's post once. You will get much better advice from the people who actually own these boats and engines.
For example, someone commented that the Cat 3116's are underpowered for these Sea Rays. My 400DA is 44' 4"LOA, and it runs at 25-26 mph at the recommended 2400 RPM. It runs about 30mph at WOT of 2800 RPMs. No problem at all getting up on plane. If you search this board you will find many people with the 400DA and SB, and 3116s getting real world better than 1mpg.
Bad information => bad decisions. Stick with people who know what they are talking about.
second this. Other folks around here cautioned me about our 410 with the 350hp 3126s and that it was underpowered. My boat does 6mph at idle, gets on plane effortlessly, and cruises efficiently and quietly at nearly all points in the rev range.