In the UK they are about to do a round Britain, say 1600-1800 mile in a group of sub-5m boats.
And it remains to be seen how successful they are!
I have no doubt that it is feasible to go round Britain in a five metre RIB, but I reckon it is very optimistic to only allow ten days. That's an average of around 160 miles a day, every day. The longest day is planned to be 230 miles, and that will be seriously hard work even in good conditions. It will only take a bit of shitty weather to really affect their progress. I am happy to be proved wrong, but I can't help wondering if they may have underestimated just a bit.
Coming back to the question in hand, I would suggest that a five metre boat would probably be OK, given good conditions. I don't know this area, and I don't know how reliable the weather forecasts are, but bear in mind that the smaller the boat, the less margin you have for dealing with unforseen conditions.