I have a Sigma 150-500 OS that I tested extensively against my Canon 100-400 IS L and TBH, in this case the Sigma won although the Canon is a much older design. The IQ was similar wide open at the long end and at f8 there was no visible difference to me. The Sigma OS system IMO worked much better than the (older) Canon IS and I was able to take a good handheld photo at 1/10sec. The Sigma also seemed to attain focus faster and more accurately in failing light, while both were equal in good light.
I also have 3 other L lenses and have no intention to sell any of these, the 24-105 f4 L, the 100 f2.8 L macro and the 70-200 f2.8 IS L are all brilliant performers and compared to other makes of lenses I've had in similar focal lengths are far superior. In essence, you will always find the odd lens that is comparable to an L lens at a fraction of the price, but they really are few and very far between. I've no doubt that a newly designed 100-400 IS L (or 200-400, which is the replacement) will be at least as good if not better than a Sigma EX or similar.