I apologize if this is a known issue, but I thought I'd mention it anyway and provide some examples.
When shooting in RAW mode on the 7D at 50fps, the recording size is 1728x694. You are then instructed to stretch the image vertically in post by 1.4x to correct the aspect ratio to a proper 1.78:1 (16:9) frame, which gives 1728x972. This is the same resolution you'd get if you shot at 24/25fps.
However, stretching by 1.4x still does not give a correct aspect ratio, despite filling the frame up to 16:9. The picture is still squished vertically. In order to fix this, you must stretch by
1.56x. But, if you do this and then resize back to a height of 972, you cut off the left and right sides of the frame which results in a pillarboxed image.
I haven't tried this on the 5D3 as the camera is out on a job at the moment. Once it's back, I'll test it as well.
Here are examples that show the result of stretching the image by both 1.4x and 1.56x:
1.4x:
1.56x: