The 10,000 naniwa gives a noticable refinement over the 8,000 naniwa. There isn't a lot between the 10,000 and 12,000 naniwas, though - both give exceptionally good shaving edges. The 10,000 naniwa after a coticule makes the edge a little bit keener - although the naniwas still give a smooth shave, just not as smooth as the coticule (with the exception of a very hard tiger-striped coticule I have, which gives quite a keen edge).
Regards,
Neil