Make you free of thinking of "slot effect".

The first sail is always in the upwash of the second one, which is always in the downwash of the first one. The more seperated the sails are the lower the drag by the way. But aerodynamic drag is not very important on downwind legs.
To increase lift the slot must be very narrow (about 40 mm to give you a number), stable and form a convergent nozzle with the mast. All of this is not present on todays soft sail designs, hence no slot effect.
Hence, as macca said, a longer pole is better (for exact the reasons he mentioned above) as long as you can steer the boat.

Another question is, is leehelm a bad thing (a bit of lee-helm, not massive)? I have the feeling that with leehelm the rudder pushes the stern down and bows up. Hence you can have abit more power in the rig before you start to submerge.

Cheers,

Klaus