In answer to your question:
Planing, yes. True shortboard, no. The boards I have used all have had a retractable centreboard which I have always had retracted. I have never had a problem getting up wind but then I have always had a nice onshore breeze (if that is the right term) so I never had to slog a long way back to the shore line up wind. I guess that means I have only had to hold my position and hence my uncertainty about getting a board without a centreboard.
When you say the Rio M might perform better in low winds than the Go 144 are you referring to just the upwind ability?
Your recommendation to get a 7.5 has given me the confidence to step up the sail size. When hiring equipment the salesman always talks me into a smaller sail, trying to protect me and his gear I guess.
Thanks for all your help, really appreciated.