The 400mm f/5.6 L is not very good with a 1.4xTC, so saying that the Sigma with a 2xTC is on a par with it is not exactly a selling point.
Ps On checking, I think you need to have the Sigma at f/16 to have it on a par with the Canon at f/8.
I wouldn't use it with the 2x teleconverter, the 300f2.8 IS II is much better with it.
However, the new sigma looks really really good if you don't opt for the 2x converter
With 1.4 converter and 1 stop down, you will have a 1 stop disadvantage to the canon 200-400, but it's still remarkable for a zoom less than 1/3 the price of the canonhttp://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=764&Camera=453&Sample=0&FLI=4&API=0&LensComp=844&CameraComp=453&SampleComp=0&FLIComp=6&APIComp=3
Then @ 300mm f4, it looks ''equally good'' as the 200-400 @ 300 f4 ( maybe slightly better than it).
Even on 300 f2.8 it looks close to the 200-400 @ f4
Which is pretty remarkable.
In case of emergency I guess a 2.0 converter could be added, but If you intend on buying the 200-400 with 1.4,
Consider getting the sigma 120-300 S and canon 300 f2.8 IS II and use it with two bodies http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=739&Camera=453&Sample=0&FLI=2&API=3&LensComp=764&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=2
The 300 f2.8@600 6.3, looks slightly better than the 200-400 @ 560(which might be slightly shorter due to breathing).
You would have a
120-300 f2.8 (pretty sharp)
168-420 f4(f5.6 imo) with ;equal' iq to the 200-400
300 f2.8 ( the sharpest, lightest , fastest focussing etc)
420 f4 ( equally good to the 200-400, when the 200-400 breathes you get some extra pixels on targets with the 420 f4 compared to it, similar or faster focussing and much lighter).
600 f5.6 (f6.3-f8 imo) , again more pixels on target than the 200-400 @560.
And instead of a 70-200 on a second body you can put a 24-105 on a third body! ^_^
also price wise, where the 200-400 is 11k euros , you can get the other two for around 9-9.5k, which leaves room for a second body! ( I think most people already have a backup body anyway)
the sigma 120-300 without tc on one body
canon 300 with 1.4 on another body
sigma 120-300 with 1.4 tc (shooting at 5.6) on one body
canon 300 with 2x tc (6.3-f8) on another body.
Im only thinking for wildlife photographers here, it may be cumber stone to walk around with
Or just the sigma 120-300 on one body ,
When the price comes down, or there is another sale ( one guy recently bought it for 1871 euros!) , I am sure to pick one up