In short, the clearance on the '18/'19 pullrod/bearing is wrong. It bottoms out before the clutch plates are actually used up. I was starting to get occasional slippage after a few thousand fairly-easy miles. Changing those two parts out for the '20 versions (46102-0648 and 92045-0937) made it behave just like when it was brand new. As far as I'm concerned, every '18/'19 should install these parts - heck, maybe it should even be a recall.
The HD springs will help hold the clutch and return the lever better. It's pretty common for these little Ninjas to have weak clutch springs to make a softer lever for beginners. I don't think the springs make a huge difference, but they do help.
I bought the whole Norton kit with the modified shift shaft and shift star. I wanted to replace any factory weak points while I was in there, since I'm planning to keep the bike a while. The only thing I didn't get was new friction plates, because I only had a few thousand miles on the stock parts, and the updated pullrod/bearing fixed the clearances.