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