Hi hrcap, Having some experience with cabin ribs your post got my attention.
Probond Marine in Buckie are now building Carson RIBs with cabins from 7.5m and above that have also been fitted with diesels (and outboards). Carsons were the first purpose designed cabin ribs from the early nineties with 9m then later 7.5m (also had a stepped hull). More info can be found on Probond's website www.probondmarine.co.uk
and also if you have a look at https://papayarcticadventure.blogspot.co.uk/
you will see Quickstep with a cabin, wheelhouse and inboard engine currently in the Arctic having voyaged from the Orkneys. You can also find more information about these ribs on this site if you search 'Carson'.
All Carson cabin ribs have proper little cabins, not just cuddy shelters, and can have berths, room for a loo, stove and even standing headroom.
I'd be happy to help with any queries.