Got both and they are equally awesome for their respective use. Now, for your question, I just tried - as we speak - to put my 65mm of extension tubes on my 70-200II just to see what. No shot taken. So, the min focusing distance becomes about 2' and the IS looks like working just fine. With that stacking of tubes, your magnification at 200mm should become 0.21 + 65/200 = 0.535
This means that objects taken with this stack will appear about half their size in the picture. This is not really macro, but it could be a good start. Extension tubes are quite cheap stuff. No need Canon for that, others can do as good. After all, these are just full of air. So, if your are not yet fully dipped into macro (the day you start, you can't get away anymore), I would consider the 70-200II and add a few tubes to it. You have the best walkabout lens money can buy and 0.535x mag for mid-macro.
Just a word more. The 100L lens is just an tremendous macro lens. I can tell you that the IS still works, though not in full, at close distance. I also stack tubes on it for like 1.5 - 1.6x and the results always blow my socks off. When you are out there shooting through wet foliage of bushes, you're happy to count on weather sealing.
So I did some more testing. I setup my 70-200 on a tripod and a measuring stick to measure the MFD of a subject at 70mm and at 200mm with and without the Kenko Extension Tubes. Also, by adding the additional magnification factor based on your math, I came up with these figures:
Attachment MFD (in in. @70mm) MFD (in in. @200mm) Total Magnification @70mm Total Mag. @200mm
(none) 39" 37" 0.21 0.21
12mm 12" 29" 0.3814 0.27
20mm 8" (manual) 26" 0.4957 0.31
12+20mm 4.5" (manual) 21.75" 0.6671 0.37
36mm 4.25" (manual) 21" 0.7243 0.39
12+36mm 2.25" (manual) 18.25" 0.8957 0.45
20+36mm 1.75" 17" 1.01 0.49
12+20+36mm 1" (manual) 15.5" 1.1814 0.55
Did I do something wrong? From these figures, it looks like I can get more than 1:1 magnification with my 70-200 @70mm. Or do I only get a 0.21x baseline magnification factor at 200mm? I initially did this test just to see what distance I should be at for AF to work. I ended up doing some more math and added those figures to this table. Can someone please clarify this for me?