At 10 mph (8.7 Knots) you probably won't plane even with an 8.5 m2 rig.
But at 12 mph (10.4 knots), with an 8.5 m2 rig you should be able to plane if you are
active, move back on the board progressively, and learn to pump.
At 15 mph (13.03 knots) you could probably plane on a 7.5 m2 rig.
So, maybe get an 8.0 m2 free race rig.
I recomend free race rigs as they have very good low end power, are light in weight and easy to rig well.
They also have a relatively wide tuning range.
If you go with an 8.0 m2 rig, you may need to upgrade to a longer 490 mast, but there are some 8.0 sails
that still rig on a 460 cm mast with a pretty long extension under it.
Be sure, when you want to get planing, to have the centerboard completely stowed up inside the board.
Rio CB's have been known to break off if you push them too hard with lots of speed.
Plus, the added drag of the CB will definitely make it harder to plane, and give you some control
issues if you happen to plane with it down.
Hope this helps,