The jib, on a rotating mast is usually attached, on the mast, at the same location as the hounds, and if you were to attach a new jib higher than that you should really raise the position of the hounds accordingly to give the correct support to the mast. If you start playing around by attaching a "working" jib above the hounds you can get into all sorts of mast bend problems. If you raise the hound position you are left with all sorts of ugly holes in the mast from it's original position which can weaken the mast in a very suspect load point, as well as having to replace all your rigging. By putting on a "taller" jib and keeping the foot length the same you would only be gaining a very minute extra area at the smallest part of the sail and I don' think that you would even notice any performance difference (and potentially if it wasn't all converted correctly, there could be an actual drop in control and performance). Adding to the mainsail area is an entirely different (and easier) situation where there should be a marked improvement in power (particularly more so from its height, and higher aspect ratio than from the extra area)