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The 6D has very little banding at low ISO. The only banding you can see is the low frequency variety when viewed at downsampled resolution. It has absolutely zero banding at High ISO, even less than the D3s.
Here is the banding performance of 6 cameras - 6D, 5DM3, D800, D600, D5100, and D7100. All images were shot -5EV and pushed +5EV (raw, developed in LR)
You have more shoes than my wife, Adam