How fast you can go on the GO 151 depends alot on the sea state.
Probably the optimum speed (over 30 knots ...55.5 Km/h) can be achieved with a
7.0-8.0 m2 rig in 16-20 knots as the water remains pretty flat.
You have good speed so far!
Did you get the 49 K/h on the 9.0 m2 or the 6.0 m2?
6.0 is getting toward kinda small for the GO 151, and 9.0 is pretty large for that board.
Perhaps something in the middle (like a 7.5 m2) will get you to the max. possible.