The Hobie SX has a larger sailplan, longer wings then the 18 Magnum and I believe a different mast in addition to the spin. The boat never caught on in the US as a racing class. Matt Miller would be the best source of the history of this boat.

If your point is that the rating for the SX is too high... that requires a look at the history of the rating. The PN committe recognized that recent data on boats that are not actively raced in a class or in portsmouth or are out of production or antiques are likely to be using 10 year old sails and the rating may have crept upwards (unfairly). This was corrected for by using the rating data when the boats were new and actively campaigned.

I don't know if the SX rating was evaluated at that time.

Take Care
Mark