Those are just clutch "kits" that allow you to adjust your weights and spring to fine tune for tire sizes, tunes, gearing, etc. The only complete aftermarket clutches that are really available are from STM to the best of my knowledge, and that's $$$. There's a few aftermarket upgrades, like primary sliders an such, but everything is mostly based off the OEM clutches.
With 28" paddles and not much else done to the machine, your stock clutching is probably pretty close still really, you might see a small improvement if you tune it just right with a clutch kit, but once you start messing with the clutching, it can open a big can of worms