I want the ultimate set up for nighttime athletics (etc.)
Currently have a D6 and 70-200 2.8 mk2 and then just crop like heck in DxO9
1. Buy a 200-400 with 1.4 tele (but it weighs as much as a gallon of milk, is expensive and then miss shots when the ball comes to this side of the field)
2. Add the 2x tele mk3 but lose some f-stop and sharpness and also lose the close shots
3. Buy a crop sensor body or use the T5i or EOS M I already have with the 70-200 and use the Canon 6D with Sigma 24-105 Art lens.
Will a good crop sensor (like on the 70D) have just as much sharpness and resolution as the full sized sensor on the 6D, or do you lose resolution because the lens can't handle the resolution, or the sensor isn't as good as the 6D even though they have managed to squeeze just as many pixels on the APS-C as on the full-sized 6D sensor?
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).
I want the best, but don't want to spend $11k if I will have just as many challengess as I've got now.
You think the price of the 200-400 1.4 will continue to drop?
The ultimate setup for low light sports is a D1X and 400mm f/2.8. the 200-400 is good for daytime use.
None of the options you descripe approach a ultimate setup. A zoom isn't always necessary, but having enough light to turn up the shutter speed to at least 1/1000 sec is optimal to get sharp images of moving players.
+1 with Mt Spokane Photography ...wrong camera and wrong lens(except 70-200 f2.8 IS II)
About 6D + 300mm f2.8 IS II if budget is an issue. Keeper might be lower with 6D, however, it nearly what you want "nighttime athletics".