Rob,
Several years ago I tried sandblasting my steel plates in a CB350 race bike with a new set of aftermarket friction plates (not Barnett). The new friction plates lasted one weekend. I am not convinced that the aftermarket friction plates were not a little too thin to begin with, as I later realized that the CB350 clutch bottoms out long before the workshop manual claims, so that may have contributed. On my BSA B50 clutch with strong aftermarket springs and stock plates, I got about 9 or 10 runs before I lost my drive. Disassembling them revealed that the friction plates were not worn out, but the steel plates had burnished slick. So it seems that a proper roughness, perhaps 280 but I would guess maybe 600 might work well. But if the steel plates are too rough, the friction plates may wear out prematurely. Also, will the rough steel plates "clean" during use or will they clog up? It might be a good idea to add radial grooves as well to allow worn clutch material to escape.
I have just rebuilt my current clutch using Surflex (?) plates and adding an additional friction and steel for this year, as well as adding a new lower 1st gear. I do have some old steel plates, but they are rust pitted. Can you still process them? If not, I will try to source some good spares, pretty easy to find, and get back to you.
Tom