dpreview has posted high ISO comparison of Sony A7S, Nikon D4S, Nikon Df, and Canon EOS 6D. All these cameras has very good high ISO performance. The Sony A7S and Nikon D4S have the highest ISO range at 409600.
You can see full comparison below:
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|Sony A7s||Amazon | B&H | Adorama | eBay|
|Nikon D4S||Amazon | B&H | Adorama | Best Buy | eBay|
|Nikon Df||Amazon | B&H | Adorama | Best Buy | eBay|
|Canon EOS 6D||Amazon | B&H | Adorama | eBay|