So, more complex than it looked - just so happened my DR Slave was giving me some horizontal stretching from anisotropy so that looked like it was working.
As I kept testing though, the DR Slave remained locked at producing the same horizontal blur even if I changed the Anisotropy (so I got a "cross shape" in the blur). Then it stopped showing any horizontal or vertical stretching at all, no matter what the aniso value was. So something is broke here :)