Well I got my M4 pistons NP3+'ed by Robar probably over a year ago, but haven't had a chance to post a review.
I've shot quite a few times since then, and the carbon still just wipes right off. Here is a side by side picture of a dirty piston and a clean one. Literally takes me a minute or two to clean one. Carbon can still get nasty if you let it sit on them, then it takes longer to scrub off, but as long as you clean right after your range trip, you should be good to go.
Pretty happy with my minute upgrade.