Actually Norton rates their water stones on the JIS scale.
For how it actually performs, check out Grazors comment - last post in this thread:
http://straightrazorpalace.com/hones...-5k-jis-3.html
Really the only 'problem' with Norton hones is the lack of anything beyond 8K. And that's not really a problem - but they are considering fixing it. People have finished on the Norton 8K and gotten close/clean/comfortable shaves. They've also gone from the yellow brick to the Naniwa 10K/12K, every natural finisher known to man kind, CBN, FeOx, Diamond and Chrome Ox, Shapton 12/16/30K hones, the Gok 20, and I follow up with a Kuromaku 12K when I'm in the mood to create a ridiculously sharp synthetic edge.
Comparing the Norton 1/4/8 to Naniwa 1/3/5/8, Shapton 1/2/4/8, or King 1/4-6/8 is like comparing a 30-06 to a pair of .308s. and a .303 Brit. Realistically there isn't much that one can do that the other 3 can't. Just a matter of figuring out which one of the Big 4 appeal to you and having at it. All roads lead to Rome.