I've done 19.8 sustained over 10 seconds according to my SpeedPuck. That was solo with spi. You're really on the edge of the pitchpole at those speed, with the boat humming and skipping all around. I still haven't cracked a sustained 20 knots, but it's definitely possible and probably easier to achieve with a newer F16 design.

We did hit 19.2knots this week end with my son on a reach with no spi and the small Stealth sails (13m2 pin-head main, minuscule blade jib). Interestingly enough we were faster reaching without spi than downwind with it.