The reason that virtually everyone models the iris concave is because that's what renders well in the typical user's hands. A flat iris usually looks awful when rendered.
For example the LaFemme eye shaders with refraction look best by far, but if you load them in with typical Superfly settings they're going to look awful. Now imagine eyes with corneal lenses being rendered at the default 3 bounces - they're just going to render black. So there's a reason things are the way they are.

That said, I'm happy to experiment with this stuff -- hang on a few more minutes ?