If the boat is balanced and fast upwind but has lee helm downhill then it's not so good to mess with mast rake or rudder rake, it is much better to move the board position forward. It will have none or super small effect upwind (In a reasonable range) but will fix the lee helm.
The slot effect is minimal compared to the increase in air to the front of the kite. There are two things that make you faster downwind:-
1. Longer luff length
2. Longer pole
If I could have a 4.5m long pole on the F18 I would. But right now I will be happy that I can paint my boats
Moving the board forward will I guess move more effort onto the board and away from the rudder, the A's seem to have such small boards that the A has to take some of the loadings onto the rudders to compensate and the rigs position becomes far more critical to balance things up a bit.
The F18's seem to be going the other way with almost all of the lateral effort placed on huuuuuuge loooooooong high aspect boards with diddy small rudders just touching the water at the back. I guess thats all to do with the change of balance of the boat when that big spinnaker goes up.
Still not convinced about the spinnaker not forming some form of slot, the Shadow used to have a mere hankerchief as Spinny run straight from the bow, it was a very fast boat considering how little sail area it had. The new X version seems to be more normal in its spinny arrangement and seems to go just as well with a larger area.
I guess one will have to give it a try some time, I guess just like being able to paint your boats ( thought that had been resolved in your favour )
