In autumn I had the chance to race against a couple of Europe dinghies (very established class i Scandinavia).
Wind was around 10 to 16 knots. I was on Equipe + 10.3 sail. Upwind the dinghies pointed better than me, but I did +10 knots of speed riding the dagger, so I outraced them quite easily.
On a short course with a lot of tacking a dinghy turns quite fast and so gain an advantage.
On a reach I could keep the dagger down and manage 10 to 13 knots easily. No way they could follow me. Had I sailed my Formula 159 I would have beat them even more. The formula does 25 knots on a reach easily. With an 8.5 rig I have done 26 knots on the longboard in 20 knots of wind. No normal sailing yacht will be able to keep up.
Again in less than 10 knots of wind its a matter of waterline length and rigsize.
I've tried to race against Tornados, Hobies and tris of "normal" size in windy conditions on a reach on Formulas, freerides etc. No way they can keep up with a windsurfer.
A formula 40 cat will beat a Windsurfer though..
Sooo.. What about Serenity in 5 to 10 knots against a sailboat??