The factory is around 300 I believe. We have a 2007 and we replaced the factory sac with the ProX Rear Seat Bottom (http://www.buywake.com/catalog/detail/FL_ttJqn). It really makes a big difference in the wake. When we surf, we also add a 800lb (http://www.buywake.com/catalog/detail/jw6SQ) next to the engine.

The fuel tank is right under the rear ski locker, and I think the max you can put in there is a couple hundred pounds. I wouldn't plumb a sac in there if that's what you meant.

Where are you located? If you're close to Houston you can just buy mine listed in the classifieds...it's ready to go