For what its worth and PacificNWrib has to decide for himself what he wants to do, I would quite happily go 15 mile offshore in his 4 meter RIB with the 50 HP on the back.
I'd make sure its a nice day and check the forecast first.
Indeed its something I used to do quite happily in my Searider when the conditions were right.
Now if the question is "What size RIB do I need to go 15 miles off shore in any conditions". Then I'm afraid I can't answer that. It would probably need to be a ship for that sort of thing.
Now where is that piece of string