Hi Pepin,
I get that you can`t change sails, mast & rudders etc as windsurfers can - I sail cats too.
I think the polars for F18 only show the obvious - that with spi on a broad reach is faster than without - have you seen polars for F18 on a beam reach ? Even so they might be slower as F18 (like F16) is designed for max. efficiency upwind and down, not for reaching. I wouldn`t be surprised if an older design with large jib is the king of beam reaching at speed.
I think that our current designs are geared more for developing lots of power with lots of drag, especially the spinnakers. Perhaps a flat-cut spinnaker would reduce the drag, remember we sail with so much apparent wind that it feels like we are beating, yet we have headsails with a lot of fullness relative to the AOA we are sailing to, which is why the big bearaways are necessary in gusts.
I think 20knots is really fast solo, but you seem to have gotten close a few times, just keep at it and you will get there.(Or explode in a ball of spray trying..)
Now your post has become a challenge.. We have a forcast of 21-25knots this weekend...