It's not as outrageous as it sounds ... really!
Based on GaryGee's recent info, the 7.8 Offshore (which is a heavily-built boat) does about 34 kts with a 240 HP inboard diesel (might be 230 HP).
While I appreciate that this is perhaps a little under twice the weight of an outboard of equivalent power output, I doubt that a 225 HPDI would produce more than say 45kts. However, I have no doubt that higher speeds would be achieved on lighter RIBs of the same length and beam.
Given that the 225, 250 and 300 HPDI engines are exactly the same weight and have the same cylinder block, I should have thought that there would be no difference in fuel consumption at cruising speed (say 35 kts). I would therefore suggest that "economy" at cruising speed is not really an issue, when considering these engines.
On this basis, what is the point in limiting the top speed (not to mention the acceleration) by going for the lower power output?
Manos - What a great chap!