So I read that I don't yet need a Canon MP-E lens if I can sort of get by using an extension tube set with a f/1.8 50mm lens with a teleconverter. It's not bad, though it would probably be crisper/sharper with the MP-E lens.

Here I used a 580EX with the little white bounce flap ou to bounce light down (thus the shadow of the eyelid). I used the 50mm lens on top of a Kenko set of extension tubes and put that on a Kenko 300 Pro 2x teleconverter. Supposedly this is in the neighborhood of about 4x lifesize. It is bigger than just using the lens+extension tubes w/out the teleconverter.

I like the waviness of the iris here. It doesn't quite show her eyecolor (hazel, so it's green and brown) though, maybe because of the level of zoom.