1.4 works fine on the 70-200 2.0 is crap though very soft at the longer reaches so beware.
They both work fine Daniel, even stacked on top of each other.
Don't know where Paul's statement is coming from but it might because he is using Nikon and i have seen earlier topics where Nikon users were not happy with the Sigma combo.
On your Canon though it is absolutely no problem at all although i would not try a Canon 70-200 on a Sigma tc and v.v.
Here are some examples for which i used a Sigma 70-200/2.8 EX(hsm) combined with a 2.0 tc.
Data: Hunter 293mm, 1/1000s, F7.1
Data: Spit 350mm, 1/800s, F6
And 2 at maximum range.
Data: Pilat 400mm, 1/400s, F7.1
Data: Connie 400mm, 1/640s, F7.1