With my 87 kg, I probably plane even with my 8.8 in 5 m/s = +/- 10 knots. For sure in a downwind slalom course.
The thing is that I also want to plane upwind in 8 knots average. That also means regular lulls of 6 knots or less.
Therefore, I prefer a 10.9 sail. It's no fun to be off and on the plane, nor to have problems to return to the starting point.
On the other side, I don't like too much freeride sails that quickly overpower me when the wind picks up. I had before a 9.5 freeride/freerace sail which horrified me in stronger winds. I'm far more comfortable with my 10.9 Formula sail than with my former 9.5 sail, despite of the heavier weight.
I guess, it's all about the range and personal preferences.
IS87 / IS101 / IS122 / US147