OK, 8-15 mph is only about 7-13 knots.
At these windspeeds, I think I'd suggest one of the new Rio's (Probably the M for a 70 Kg. (154 lbs) sailor).
Since you rartely have over 12 knots of wind, and you have learned to use a centerboard to stay upwind, the Rio M (or even the Rio S) would give you the most time on the water.
Smaller or wider boards, without a centerboard, are good above 10 knots (11.5 mph) but you would be slogging along in < 10 knots, even with a big rig.
I've had the new Rio M out in 18 + knots on a 6.6 m2 4 cam Sailworks NXslm and it was a very nice planing ride.
Not the first board I would choose for this kind of conditions, but I'm sure much better when the winds are < 10 knots and you want to rail the board upwind on the centerboard.
The GO or Futura (as suggested by Crazy Chemical) would be real good in 12 mph with a 7.5-8.5 m2 rig, but how much of the time will you get the higher winds?
Even a larger and wider GO/Futura and a bigger rig would be OK, but slogging in < 10 knots won't be much fun.
It seems that planing is what you want to do the most, and that's simply not going to happen with the windspeeds you have and anything less than a 7.5 m2 rig.
For your suggested windspeeds, an 8.5 m2 would be even better.
Are there other windsurfers on your lake? Do they get planing? What size boards and rigs are they using to accomplish this?
Doesn't sound like you are ready for huge sails and formula gear yet, so perhaps the suggested Rio M might be the best overall compromise for you.
Hope this helps,