after some thinking about sag (and i must admit some initial scoffing) i increased the preload on my stock spring such that i have about 5mm free sag and 35mm sag and it made a huge difference. it turned out to be about 5 turns on the nut so my preload was pretty far out of adjustment. the mushy floaty feeling is gone and i can shove it into a corner hard and and roll on and it behaves nicely.
so....you want the right preload so the rear end doesnt have a lot of movement that disrupts the handling when you go from front to rear weight transfer
I thought someone had posted the different spring rates, but I can't find the thread so maybe it was the front ones. From extensive riding of a 2004(750) and 2008(910) that the later bikes appear to my back to have a softer spring. It might be the damping that is different but the spring is definitely a different colour.