Great find with the dissections bagginsbill! 

I must say that I feel somewhat vindicated.

The dissections show the shape very clearly. Not exactly my shape, but not far off given the constraints of the mesh.

If I stand in front of a mirror in a darkened room and shine a flash light at an approximate right angle to my eye, I see the side of the iris closest to the light catching the light, except where it dips in towards the pupil where it is in shadow. I see the other side or the iris as it curves out from the pupil lit strongly then fading away to shadow as it curves back in towards the sclera. This is all as I would expect from my understanding of the shape. I don't see anything that would suggest an overall concave.

@ MikeJ,

Your image clearly shows (as did one of bagginsbill's) that refraction can counter concavity. In fact I think there is too much refraction in the "Convex-with refraction" image, and that a real iris does not appear so close to the cornea. However I am still not convinced that there is any good reason to use a concave iris, when a real one is convex. I will stand to be corrected on this point, but I have not seen any convincing evidence so far. Cobaltdream posted some links to good photos of eyes, and seems to think they display deviance of a concave appearance. I look at those same photos and see nothing but evidence of a shape that is roughly a convex toroid.