Michael, I'm curious - your 100mm macro goes 1:1 magnification by itself - why add tubes to it? I may be wrong but your images don't appear to be greater than life-size.
Hey Loupey,
I finally made some time to take a few shots with the 100mm macro with and without the tubes. Here are a couple shots of the same subject. It wasn't a scientific experiment, but this is as close as I could get with the 100mm macro by itself, and one with all 3 tubes.