A bit of a curve ball on the subject.

I have a friend who owns a sigma 2.0 & 1.4x TC, the pro ones. As a bit of a muck around I borrowed the 2.0x and tried it out on my 70-200mm F4L also with a tamron 1.4x I owned at the time and compared it to my Sigma 135-400mm. I tried both set up's off a manfrotto though back then I was shooting with a 300D.

Bottom line was there wasn't much in it. The canon spanked the sigma in the relevant ranges without any TC and with the 1.4x TC (in the up to 280mm range). But when the 2.0x came into play the IQ degradation on the canon made it pretty comparable to the sigma across the board as both are similar focal ranges or 140-400 v 135-400mm.
If you consider the price you can pick up a lens such as the sigma 135-400mm compared to a teleconvertor, it could be a good choice to use a fairly cheap longer lens rather than a teleconvertor and degrade a superior lens down to the same level of IQ.