Eric,

As I understand it the jib bends the wind around and 'funnels' the wind across the back of the main but it also has the effect of causing the main to see a header. So that the direction of the wind as it meets the main is headed somewhat on a sloop but of course on a cat rig the main sees the wind without the header caused by the jib. This has some effect on the pointing ability, I suspect, of a sloop v a cat.

Rob.