There was a Phantom 28 for sale last year with a 345hp diesel and surface drive IIRC. Looked like a good fast economical boat (of course albeit not as seaworthy as a RIB
). Is that your boat?
On the performance question I am convinced personally that outboards are the way to go for smaller (say 7.5 and less) RIBS where useage is around 100hrs per season. Arguably outboards are better suited for purpose than heavy diesel engines in relative light RIBS (hence mounts cracking etc) and overstressed drives - Bravo 3/3x/Yanmar 300 failures being the case in point.
Fuel availability can be an issue but most RIBS of this size live on trailers and can hence be refuelled at Tesco's.
Bigger RIBS its a different matter. If you need to refuel alongside and do extended cruising then diesel is preferable. I do not believe however that a truly great engine and drive combination exists for the 8m+ RIB. Upto 250hp (Yanmar 250/Bravo or Yamaha 240/Hydradrive) but over 300hp?
For Seahound the choice is clear - same engines and outrageous fuel bills. New outboards at some cost and much reduced fuel bills (probably in the order of 40%+ compared to race tuned engines?) or effectively selling the boat and buying another one with diesel engines!
All IMHO, FWIW etc etc.