Glad to hear you are still happy with the Rio L.
At first I didn't lke the feel of the domed deck either, but after sailing the Rio M a bit more, I found that if I get my heel slighty out on the flat area at the bottom of the dome, I can sail the Rio fully planing just like any other board.
It just takes a while to get over the different angle you have to place your back foot at.
I'm not the guy to ask about SUP.
I have a 12' 6" Wood SUP as a demo board, and I've actually "paddled" it a few times, but mostly I put a 5.0 Retro Ripper or a 6.5 Retro on it and sail it.
I would think having the CB down would help with "direcionality" but the drag might be significant.
Hope this helps,