In the end we did get the Achilles HB270 LX
folding transom RIB. We may end up regretting the size but at the moment it feels fine. It 'happily' carries my wife, myself, our 270lb nephew and some gear and it planes easily with just me onboard at half throttle (still running in the OB).
The folding transom is great. It packs down really flat (even after inflation...) and goes comfortably on the front deck without blocking the front hatch. We sailed back from Westport Pt. with it mounted there and it was no trouble at all. I know... The life raft is not going to work too well from under it but I'm not at all convinced a life raft makes sense for coastal cruising anyway so it might be coming off until we do some 'serious' voyaging and I see davits in my future for coastal stuff.
Not sure about the boat bag. It keeps the RIB somewhat protected but it is a royal PITA, particularly if there is some wind.
Thanks for all your great input and advice and apologies to those we ignored......