Going fast is no easy thing. Lots of people do go very fast, but it takes the right board, sail, fin, wind, water state, venue, skill. I weight the same as you, but have a iS 111 with a 42 Tetonics Tomcat fin. With a 7.6 race sail, I have hit 31 knots on my GPS twice, but I can't seem to go any faster. I have been carrying a GPS for the last 8 years on every outing.
I am not heavy, my sailing site only allows an occasional run into flat water with good wind and the board & fin are too large for really fast speeds (35-40 knots). I have the skill (28 yrs windsurfing), but have hit a wall with my set up.
Your 7.6 may be a little small for your board, but given your weight, I don't think you will do any better with the 9.2. An 8.5 may be what you need.
Good luck, it's fun to chase personal speed records.
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.