Hey Notwal,
I've done 35 knots on my 5m Ezzy wavesail so speed sails aren't essential for going fast...
Rig big, lots of downhaul, bag the sail out. It feels like a big sack of potatoes but when you're going off the wind it's fast

Also put your harness lines back further than you normally have them, you want to sail along with lots of front hand pressure. This helps to prevent spinout at speed, which can be quite frightening...
And join the challenge, you know you want to