I think your 8.5 is too small for 9-15 knots. If you really want to have fun, you will need at least a 9.2 or 10.0. They both will work great on the Ultrasonic.
While the iSonics carry large sails and plane somewhat early, they are not as much fun for recreational sailing as the Futura or the Ultrasonic. For your wind range, the Ultrasonic is the call. With a 10.0 and some pumping, you will get planing at around 8 knots.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.