They are 3-5 micron particles (spyderco says that either in their literature or on their page). They say that, and then they say in different places that the same abrasive is used in the different stones, and that the UF and F are the same except for the surface finish.

The binder cuts, too, so it's hard to describe how they really behave without describing the matrix that holds the particles.

the UF cuts, in my opinion, finer than 3-5 would suggest after a very short period of time. the medium cuts very very fast at first and then settles in to cut something like a 2000 or 3000 grit waterstone would.

You are right that diamond honing the stones can wake them up again.

Mike - I'd leave the mill marks in the stone for now and use it for a while. They may be a little bit helpful in keeping the stone cutting without necessarily affecting the finish much. Once you get rid of them, it's difficult to get them back.