I started out with the Jap stones just for the money sake. When talking with FireStart, he told me that if I bought them they take a bit more work on the razor, as they don't cut as well as the Norton Combo.

Once I bought the Norton Combo, I could use the Jap Stones (4K, 8K) for light finishing on the blades edge. This seems to work out very nicely. So, if you are strapped for cash, then the Jap stones will work just fine, though you need to modify the pyramid a bit. It will take a bit longer but give good results after you strop it as well.

Hope that helps.