I've got 750s in the rear lockers, stock KGB, and an IBS under the front seats. I fill the IBS until the front seat cushions and cupholders start to "float". I've got it plumbed off the KGB pump with a 3-way valve to direct which bag the water goes to.
Not sure how much weight I get out of it, but it looks reasonably full. You can probably get more weight with bags that don't live under the seats, but I use seats for people and I really don't like walking over/around bags on the floor.
2002 X-Star w/ LTR