as far as i am aware its more to do with the body that is being used ... I tried a 120 - 500 sigma with a 2x on a 300D and it didnt want to know (at least the ttl and af didnt work, manual was fine) .. but when i put it on a 30D it worked a treat ...
find a london camera exchange, imo they are very helpful at letting you try these things out ... some have canon pro centres so they may even be able to tell you over the phone .. there is one in Lincoln if it helps ...
01522 514131 9am - 5.30pm