I think relative to other Tokoname potters, he's relatively well known. But what you're really paying for mostly for the 300 ml one is the waka engravings on the side of the pot by a calligrapher.
I don't think comparing it to zini is the best comparison. I'm no expert like a like of people on the forum on Chinese clays, but in my mind there really is no Japanese clay equivalent of Zini/Zhuni. I think it would closer to compare it shudei with hong ni. Aesthetically, and in mineral content(both high in iron, and i'm sure in other aspects). Zini tends to have a "sandier" consistency to it as,well.
In terms of rarity... since there isn't much about hon shudei in English, all I've really heard is that Gisui is the only potter still around still using the clay. Due to changes in crop-rotation techniques, taking that specific variety of clay from rice paddies was no longer viable. And so the clay disappeared. Again, this is all I've ever heard and read about it( and from one vendor unfortunately
) so there may be more to this then changes in agri.
Hopefully this helps a bit. I'm unfortunately a noob still to all of this