The Canon convertors are built to the same "L" philosophy and are designed to match the "L" lenses. Sigmas aren't.
Sorry Colin but this is not correct.
Maybe you are mixing up the Sigma consumer stuff with the EX range.
The EX is the professional line of Sigma, the Canon L competition so to say and does have a dedicated converter which will match with the EX only.
There for the difference in price isn't that big at all.
In case of the 70-200/2.8EX(hsm) you are looking at 200 Euro or so of course compared to the Canon 70-200/2.8L
I wasn't trying to say the white glass isn't good, it is very good but that doesn't mean something else is bad as you clearly understand.
Just could not resist the temptation to get back on some biased white glass users. hehe