The difference in the set up of a cat rigged mossie as compared to the sloop was pimarily for the reasons that the cut of the mainsail for sloop was wrong and cut to full for one up cat rigged the diamonds and rigging as well was, from memory set differently. This still kept the class "legal" but it made sailing cat rigged much more "comfortable". They were a very nice cat to sail one up (after all thats the way they were originally designed) and I have fond memories from them (even with the early propensity for them as sloop to fold the bows inwards under the extra loads of the jib and a crew) Still we soon sorted that one out.