5-15 knots it's perfect range for formula board. But learning gybing is a little difficult on formula. Another problem is planing with straps, because they are in extremely outboard position. And third problem - you will need bigger (about 9,5 m) freerace sail to start sailing on formula board. If you choose this option, after improving your skills, you'll need 11-12 m pure race sail.
I suppose that better way for fast improving your skills is to buy big freeride/freerace board, for instance Starboard Futura 155 or Go 155, unluckily they are not offered now. These are very early planing boards (but far later than Formula) and easy to progressing. Of course you need minimum 9,5 freerace sail for light wind days.
In my opinion Shark is good board for begginers, but start planing on it in light wind is rather difficult. If you want pure freeride board, choose for instance Starboard Carve, JP X-Cite Ride or similar. Sailing on chop/small waves will be a little easier than on Futura, but you need another 2 knots to start planing (compared to Futura).
I hope it help, altough I'm not member of the Team...