i've no experience with adding jibs to uni cats but on your question of oversized jibs i seem to remember reading that some people have added nacra 5.8 jibs to the nacra 5.5

from that i'd guess that a jib from a prindle 16 or hobie 16 would be around the right size and pretty easy to find 2nd hand

have you seached for pics of both and compared their rigging to your prindle 15