I always like to underestimate rather than exaggerate
I know that for about 10 mins off Puffin Island it got a bit too exciting for me as well where waves were higher than the length of the boat (which is a shade over 19ft!) and very steep where it took a bit of work to make sure we didn't get turned over. We did the last part from the Island to the fairway entrance at approx 6.5knots as we just couldn't go any faster without launching into the air off the crests.
I have been told conditions were slightly less inshore but without knowing the area too well I was trying to keep to deeper water hoping the waves would be smaller and not breaking as the seabed was getting shallower. We weren't the furthest out though, at least one RIB was a good 1/2 mile further out than us.
I have been out in larger seas further out but not in a RIB, its usually something a lot larger and with liferafts and safety boats larger than the RIB.
I did try and push buying a bigger RIB while we were in the middle of it but she changed her mind once we got to shore later