I have a '03 919, originally equipped with preload only shock.
I went from that stock shock to an '04 shock (eBay). Preload and rebound damping. Some improvement.
...Now have gone to the Progressive Suspension 420 shock. This is a big improvement over both the stock '03 and the '04 replacement. PS says it is no longer being made, but these shocks are still in some supply-house inventories. I got it from Motorcycle Accessory Warehouse for $315.
My hypothetical next move would be a Penske, but I need to ride more on the current set-up to see if I want to try it. I am a not-incredibly-demanding older rider, and having arrived at much better than stock now, am unsure how much better beyond this I want/need to go (trying to retain some sense of realism concerning abilities, needs, benefits, and costs)...