Oh yeah, wanted to add my boat data to the thread that Rolf locked :
My boat weighed in at 99kg sloop without spinnaker. Rough guess 105kg with kite, it hasn`t been weighed with one. My mainsail and jib are a very heavy cloth to withstand Cape Town weather, we don`t replace sails too often. I`ve weighed mainsail, jib & boom at 20kg, could lose 8kg easily. (boom is a alu. dinghy mast section, at least 5kg.) Of course I have 12,5sqm main on a 7,3m mast, is my boat F16 compliant, yes if I carry 2kg of lead. It won`t "rate" as fast as full F16, but will still win on line in over 25knots against a lot of "faster" designs with taller mainsails and bigger kites. Unless the sailors on the more powerful boat are a LOT better than I am.
The Mozzie with kite costs around 6000 Euro (new) and is only 10% slower (on paper) than a full F16.

As a separate issue, I didn`t know the Spitfire was "un-grandfathered" It`s an interesting issue, as it weighs a LOT more than min. F16 weight (135kg I think), but has a slightly bigger jib & spinnaker, the only differences if I`m correct, and it`s ISAF rating is so similar to F16 that it was initially included. I raise this because it`s a valid argument that a well-designed boat over the min. weight can be fast enough to be rated at a similar enough ISAF rating to allow it to compete with F16`s on the water in real terms, which is what F16 is all about, not "how heavy is your tiller extension" I don`t know much about the Viper, but if it`s similarly designd with respect to hull volume to the Spitfire, it might just surprise a lot of people.