Hi Russell,
I think I know what your cat looks like, it is likely your Calypso cat would qualify for F16. If you want to make sure you can check the meausrement rules at
www.geocities.com/F16HPclass/. As standard your cat is unlikely to have any problems, but be careful with the size of your new sails if you are serious about racing F16, it could be possible to make them too big.
You are certainly welcome to join the F16 group, many of the class in OZ are Mosquito's which where designed before the Caper cat so your boats age is no problem.
As you say it may be heavier than minimum, you can get a rough idea by using bathroom scales, weighing your boat with all the rigging on it on a level surface one hull at a time and add both figures together. Assuming it is over minimum weight you comply with the rules, there is no maximum and alot of F16's are over minimum, but as you say it's competitivness will be affected if it is to heavy, but not it's eligibility to race.
Keep an eye on the Forum for F16 events in OZ would love to see you at one, racing will certainly be the quickest way to develop your cat to it's maximum potential.
Regards Gary.