Is it about what configuration it's closer to or what number is fair? Being that a competition handicap system is to try and make competition "fair", it should be the latter. You are basically giving the boat a free spinnaker power-up by using the SX rating.

In light stuff, a well sailed Hobie 18 can keep pace upwind with F18s and the below the waterline shape is similar to the Tiger but with much fatter boards. I haven't sailed along side a TheMightyHobie18 in anything else yet.