One of the reasons I really like the Tam is that the jump from 4K to 8K is rather large. The second reason is as Josh mentions, a softening of the texture on the bevel, the Norton 4K is very effective for me but the scratch pattern has sharp ridge lines that I want to soften and the Norton 8K does not do that as well as the Tam. BTW, I normally use 50 laps on the Tam with slurry.

The Tams characteristic as being good for microchipping blades was proven again just yesterday. I was honing a 6/8 Puma that was not behaving well and the Tam took care of that problem. What I think is happening is that for some brittle steels the jump from 4K Norton to 8K Norton is simply to much.

I think so much of the Tams that I have two from Simon Coul and a bunch of other pieces that I picked up elswhere.
Of course it is also a natural stone and one that will soon not be available anymore since the mine has been closed.

Just my two cents,