Hi Tony and James,
I had the '08 Rio M out today on freshwater (Cedar Creek Reservoir SE of Dallas, TX)
I used a System B custom 58 cm race fin (this fin was mfg. on Maui in 2000).
It's got a very powerful CNC foiled profile.
I had no trouble getting planing with a 7.2 m2 Sailworks NXslm 4 cam race sail in around
14 -16 knots of wind.
I had no trouble controlling the 58 cm fin even in the gusts.
So, I'm not sure that you need to go all the way to 58 cm, but I'd suggest 52-56 as the
sweet range for fins on the new Rios.
The OEM provided 41 cm cleaver fin is OK, but once you get a rig larger than about 7.0, an improved fin will really help with getting planing and being able to stow the centerboard and really get the best planing performance from the Rio.
Hope this helps,