Hi Wouter,
I don't want to get rid of the component of jib sheet tension that tends to center the jib car. With a fixed jib sheet, when the car moves in, the jib clew moves up slightly and the leech tension is eased and therefore it opens slightly. The slot does not become closed off. Actually the set of the sail changes very little because the pressure of the wind on the sail is down and the sail is not trimmed as tighly at the same time.
Another question: When you say "curved track", do you mean a track bent to the radius of the foot of the jib or do you mean a radius greater than that but less than the radius of a straight track?
Bill