Never will be a Sigma lens better than a L Canon lens.
You made a very short turn here mate.
Some of the Sigma's(EX) are very close or even just as good as there rivals from Canon which is discussed quite a few times already so i am not gonna spend more words on it.
Another option might be the Sigma 80-400EX OS
(with stabilizer), a little more range then the 100-400L, cheaper but probably heavier too.
From what i have heard quite good too.