I've been using a Capreo/11-speed hybrid for a couple of years now.
my favorite cassette is 9/23, 11 speed.
Frankly, the 10T sprocket gets quite a bit more use than the 9T, and, probably more in the 11/12.
If you look at the Capreo sprockets, you'll see the indexing teeth, and a thin band. Remove the extra band and spacing between sprockets comes out pretty good.
I haven't had any issues using the thicker 9s sprockets with 11s chain, although I think the worst shift point is the interface between the Capreo and 11s cassette (9,10,11 from Capreo, 12 from donor).
More notes by PM if anybody is interested.