Go fish said it for me. The new boat will handle bigger seas than the crew can. Runing in it is one thing. Trying to fish and balance is another. Period is key for running in big seas. You can run in 20'ers if the period is high and it's huge swells. 5'ers will beat you to tears if the period is short enough.
The other thing is most people are poor judges of wave height. I hear about 5-6's all the time. Thats a BIG wave in a smaller "trailer" boat. I've always figured if I'm standing on the deck and I have to look UP to see the tops I'm in 6's. (insert short joke here

) A recent post on another site had a guy running 30kts in 8-12's in Buzzards Bay in a boat similar to mine. I wasn't there, so I'm not saying it didn't happen, but I'm guessing the wave heights were overestimated a bit.