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When you are shooting next to someone else with let's say the Mark IV, the noise might be lower but you've got better resolution. Side-by-side, if you took images from each camera and cropped them identically, the Mark IV will require greater cropping since it is a 1.3x sensor compared to your 7D with a 1.6x sensor. Because it is being cropped more significantly, your apparent noise level will increase more with the Mark IV. Once the image is cropped identically, the differences between both bodies is not that great. Slightly lower noise with the Mark IV, slightly higher resolution with the 7D. Post-processing the images using noise reduction + sharpening reveals almost identical image quality.
Now...if you can move closer to the subject, the Mark IV is the clear winner, but when comparisons are done from the same distance, the additional cropping required from the Mark IV virtually equalizes the image quality from both cameras.
The 1DX (in my opinion since we really haven't been able to analyze raw images) will give you lower noise but less resolution. For sports, it may not matter but for feather detail on a bird, I suspect it could be questionable as if the 1DX has enough resolution.
I would wait on the camera until months after the 1DX has been thoroughly tested before deciding on a pro body camera.
As far as the lens is concerned, if you main objective is sports, I'd go with the 300/2.8L and take along a 1.4x TC and a 2x TC depending how close you are located to the action.