I don't know about the 100/2, but with my 65mm worth of Fotodiox tubes, I can get a .88x magnification on my 85/1.8 and it does everything else better than my Canon 50/1.8. That's a 25mm subject with my 22mm sensor. That's not horrible for macro. For reference, the 50/1.8 gets a 1.3x magnification with the same set of tubes, but that is the only thing the 50/1.8 does better than the 85/1.8.
OK, thanks for that. I figured out where I went wrong. The 'useful'-ness of extension tubes is dependent on the focal length of the lens, not the MFD. MFD is totally irrelevant in this case. It makes perfect sense that you get twice the 'macro' out of using the tubes on the 50mm as opposed to the 100mm. I apologize for my misinformed post and thank you, sir, for setting me straight.
EDIT: Original post edited.