I'm one of the guys that added one on the moomba forums. As far as I know that sac is 600lbs and mine fills completely with out pushing the seats up. I'll bet you get at least 550-600lbs in there. Makes a huge difference surfing and wakeboarding.
Heres the link to a complete how-to on integrating the bag to your existing system. You will need to sign in to see pics.