I never sailed a 9.5 camber sail with as low as 8 knots with my surfboard, so that I don't know.

Personally I find sailing with 8 knots on a cat more enjoyable then (what I think it is) to sail on a large surfboard.

I remember me trying to surf with large sails, I even bought once a 10 meter sail. But I dumped all that stuff within 2 trials.