I,m 67Kg and my experience with FW 147 is with 6.5 GTX and 8.5 Overdrive Severne, in winds from 7 to 15 knots.
IMHO big sails are for racing or very little wind conditions. If you´re not into racing than 8.5 will do just fine.
Sailing with some buddies I notice that with bigger sails they point much higher than me. Usually we start to plane at almost similar conditions. I guess being lighter with a smaller sail makes up for.
With 6.5 I find that the fin doesn´t release and I have to place my back foot off the strap towards the boards center line to have it going.
I think the sail cannot generate as much power required by the fin in light winds. Maybe you could try it if you experience the same thing with your 8.5
So, I´m not sure if it worth expending such amount of money to get a 10.5 rig compared to what you´re going to get....unless your competitive nature makes some call
My two cents