I don’t have the Naniwa 8k, but I have both the 10k and the 12k (modern white version). The only 8k that I really am OK with the edge from is the Shapton Glass High Carbon (HC, the grey one). There’s not a lot of difference in the edge feel between the 10k and 12k Naniwa. I do not believe that they are the same stone though for whatever reason, the 10k is softer and polishes well, much like folks describe the old 12k which was pink I believe.

I did a comparison (here I think) between the Naniwa 10k and the Suehiro 20k, and the difference in the edge feel was difficult to determine, the S20k edge did marginally better on some parts of my face, but overall the difference was not much. I think you’ll feel a difference going from N8k -> N10k, but that could be dependent on your beard hardness too.