Bart,
Why don't you ask Frank Meldau, the designer and manufacturer of the Cheshire Cat? Website is http://www.intl-fiberglass.com/boats.html

What I do know is that there is some reason why sailors go to a lot of trouble to get the tack of their jib down as low as possible, so you probably do not want to mount your current jib higher.

Also, with a sloop rig the main and jib have to be designed as a "team." The draft on a sloop main is graduated progressively farther aft down the sail to accommodate the overlap (or slot effect) of the jib it was designed with. So if you just arbitrarily decide to put a taller jib on the boat, the draft on the main might not be correct at that height, which would totally screw up the air flow over the main.

I'm sure the engineers and sail designers will correct me if I am wrong.