Here's what I'm talking about.. The winning goal in overtime the other night (Baylor vs Rice) from about 60 yards away, after being cropped in DxO9. Shot with 6D and ef 70-200 f/2.8 mk2 at f/6.3 1/320 ISO 12800. (Yes, I know f/6.3 was a poor choice, should have gone with f/4.5, my bad).
You think the price of the 200-400 1.4 will continue to drop?
The settings you used for this shot were indeed not the most appropriate. I would use iso 6400, aperture f/2.8 but would increase the shutter speed to 1/800. A shutter speed of only 1/320 is not sufficient to get tack sharp images for football. Be aware that you would need AFMA for that combination to.
Personal I would on my 1Dx start with shutter speed 1/1000, aperture 2.8, auto-iso and EC of +1/3stop.
A 200-400 lens would give you no better photo in this case as you already loose 1 stop caused by the difference in max aperture. I regular take photo's of sports. However, even that every one says the 7D is a great camera for sports, I must admit that my keeper rate over there is low. With the 5D3 and the 1Dx the keeper rate is above 90%. This is all caused by the more advanced AF system for those 2 cameras. I never used a 6D myself, but when I look to several specs and remarks of this camera, I can guarantee you that your sports photos might only get a big jump in keeper rate by buying a 5D3, 1Dx or the 7D2 (if the rumors are correct for the AF system). This with the right technique might increase the quality of your sport photos.
Don't start thinking about adding a 2x extender. The 2x version III is marvelous on several big whites like 300 2.8 mkii, however on the 70-200 2.8 mkii, the result is not that great. However you can use the 1.4x extender.
We will see soon a new 100-400 mkII based on the rumors. This combined with the 7D2 might be a real winner for the hobby sport photographer. I would just wait a few weeks until all the announcement for Photokina 2014 are done.
I'm sure the 200-400 will drop in price, and for sure if there will be a new 100-400. Looking at Nikon, they have already several years a 200-400. Now, after the new introduction of the 80-400, a lot of people do even let this lens at home because of the high quality of the 80-400. If Canon brings a new 100-400. It will be real good. So, the 100-400, will place some commercial push on the price of the 200-400, and I'm quite sure, that the price of the 200-400 will further drop in that case.
I would not buy a 200-400 1.4x for placing on the 6D. I rather would buy then the new 7D2 which will give you already more reach for your 70-200 2.8ii. If reach would be not sufficient, then you might add a extender 1.4x or looking to the 100-400 (mkii) of Canon or perhaps the Sigma 120-300 2.8 or a Canon 300 2.8 mk ii.
If nighttime is that important then I would advice the 1Dx body. That combined with the 70-200 2.8 ii are the max for sports in dark conditions, of if you need more range then the 300 f2.8 or the 400 f2.8.