Get the Ultrasonic...... put your 8.5 Overdrive on it (rigged full).
THEN decide if you want/need a bigger sail.
I've sailed the US 147 with 9.1 Sailworks NX fw and 8.5 Sailworks NX 4 and found that I can
get fully on plane in < 10 knots. I weigh 77 KG.
Can't stay on plane in lulls < 8 knots with the 8.5 m2 NX4, but can on the 9.1m2.
You may find you don't need > 9.5- 10.0 m2.
It is hard to find sails with good low end in > 9.5 m2 as most sails that big are designed for high wind
The Overdrive is better as it's more Jr/Youth medium wind oriented.