I've got a self-tacking jib on my Nacra 5.8. The sail area is the same as the 5.8 original jib (longer luff, shorter foot).

In moderate to heavy air, the boat goes upwind slightly faster then it used to. In light air, it is slightly slower upwind. I'm pretty certain of this, as I've been racing against the same people for many years, and am able to judge my performance relative to them very well.

Off the wind the boat is a much slower without the kite.