They finish a razor very well in my experience, very keen edge. It's a little harsher edge than a coticule though, for sure. The "curved lines" you see when there's a little bit of swarf embedded in the surface are simply the grind marks from when the stone was produced. The stones are ground flat with a diamond wheel after they are fired.

They can be lapped to produce a bit better edge, but personally I wouldn't recommend using a diamond plate. I used SiC loose grit to do mine. If you get it smoothed out and nice and flat it gives quite a nice edge, but won't do a whole lot of cutting - so you'll need to use at least an 8k synthetic stone or something equivalent before going to the UF.

The Norbit stick is an interesting idea - are you a machinist? I have a few of those in my toolbox but have never used them for much other than touching up a bench grinder wheel.