Hi Jim,
First thing, I HAVE used both lenses, so my views are based on fact not hearsay. And I'm guessing you haven't actually read my blog - that's OK, I'm not offended, much
I don't disagree with your comments. I have some cracking shots with the Sigma, some of the sharpest I've got. I'm sorry to see it go (in fact I still have it! Currently under offer from someone I know. Might be offering it here if it doesn't go). So IQ is up there for sure, but not quite as good as the Canon L Series based on my assessment, which was the question asked by the OP (but it's pretty damn close).
I haven't tested the IS back-to-back but can believe what you say is correct. The Canon's design is quite old so the IS is worth only one stop I've read - it's not a big issue for me, shooting things that move fast I need good shutter speed, so the extra stop of the Canon helps in this regard. [I'll qualify this since writing the blog - with the 7D I'm now starting to shoot some video and having good IS definitely helps when hand holding. Now, are you going to get good hand held shots with any zoom in the 400-500mm range with any amount of IS - don't know, but this may be a factor going forward - another excuse to upgrade ... :shake: <-- The Wife!]
You're also absolutely spot on about the Canon lens not being any good over 400mm - but then as a 100-400mm zoom, getting that extra out of it is quite hard

Again, if you'd read the blog you would have seen I compared IQ at Sigma 500mm and Canon 400mm cropped to the same size (so greater crop on the Canon) - Canon IQ is definitely better - I was going through exactly the same dilemma as the OP (but rather than rely on opinion I splashed out the cash to get to the facts).
Bottom line - Both are cracking lenses and neither would be a bad choice. I would (and do) highly recommend the Sigma, but my back-to-back assessment is the Canon has marginally superior IQ, even accounting for the loss of focal length, and overall
for what I use it for the Canon performs that little bit better (focus, speed etc.). If it wasn't the Canon would have been up for sale and gone by now!
Andy
ps - you're obviously a man of fine taste - 7D and Sigma 150-500