I've owned and rented several Canon 100-400 IS L's, and found it's a luck of the draw with how good a copy you have, which is why I don't own one now and am waiting for a replacement,.
I have the Tamron 70-300 VC which replaced my Canon 70-300 IS USM. While I can't swear to optical sharpness, the stabilization on the new Tamrons is more effective than the old Canons. Since I shoot handheld or w/a monopod on the bigger lenses, I find that having a good IS/VC is most important, eg, unsharp images are mostly due to movement. I'm hoping that the new Tamron is on a par w/the 100-400 and superior to the Sigma offerings, and it's a go.