Tubbies are essentially hull extensions that mount to the front of your ski making it wider and adding an extra chine. They make the ski really dig in when you turn as well as add stability by making the nose wider. As far as what resin brand to use, I don't think it matters too much. I 've used U.S. Composites, West Systems, Blowsions, and recently I've been ordering 3 gallon kits off ebay. Some are thicker than others, and some set up faster (but a lot of that has to do with temp and correct ratios). U.S. Composites has a lot to choose from but are pretty informative if you call them.
As far as other handling mods, you can shorten the hull. I took 2 1/2" off mine. It's makes the ski a little more twitchy in the back but the ski turns a lot tighter and wants to jump out of the water.