Jean-Marc perhaps you and I were separated at birth - we both like the same kind of boards. (Hypersonic and Serenity). I've sailed my Serenity 14 times now, with sails from 6.0 to 8.5, but mostly with 7.4. With the large fin the 8.5 was really unecessary. I don't know if Serenity "planes", yet it can move very fast riding mostly above the water, on the fin. In that mode, the board is challenging, and even sort of scary. More fun is lighter winds (say 5-10 mph) when it can really fly and be a real pleasure to sail with 7.4.
As for 10-15, it can be sailed in that range, though I think a conventional board would be more fun.