What mast do you use in your Severne Focus 6.0 m2 rig.
It would seem to me that you perhaps are not getting enough windspeed to get the Rio M (what year is your board as there is quite a difference between the earlier Start M based Rio and the newer '08/'09 Rio M which is a great board, but not like the Start or GO) going.
I would think at you weight, with about 13-15 knots of wind you would be planing fairly easily with the Focus 6.0 on the 08/09 Rio M with the CB retracted.
Moving up to a 7.5 is a very good way to bring your planing threshold down by about 3-4 knots.
Are you moving back on your board so it can plane freely?
Are you trying to plane with the center board down/deployed?
I really like the 7.5 NCX (I had a 2008 NCX 7.5, and it was easy to rig, very powerful,
and really easy to handle rigged on a 100% Redline 460 mast) , but I'm sure the Element 7.5 would work equally as well.
Also, as wiindz has asked, are you using a harness yet, and are you back to the footstraps.
Perhaps you aren't getting far enough back on the board to really free it up so it can
Hope this helps,