A99 tested on DXO - score: 89

The difference between 1815 of the 5D MkII and the 1555 of the A99 is not substantial, less than a 16% of a difference.  Per DXO mark, a 25% difference in score reflects about a 1/3 EV difference in ISO performance.  So you are talking not even a 1/3 EV difference, which is hardly noticeable.

For a 1/3 EV improvement over the A99, the camera would need to score around 1944.  If I did my math correctly, for a 2/3 EV improvement over the A99, the camera would need to score around 2430.  And for a full 1 EV improvement it would need to score around 3037.

But the high ISO is only part of the story, you would also have to look at how well the dynamic range and color tones respond at the varying ISOs.  Currently, it is difficult to see an overlay for comparison because DXO's site is not fully updated yet to compare the results with other cameras.

But considering the translucent mirror is taking away light, the results are pretty good.  You need to remember to look at the camera as the overall package.

Also, I don't think Sony has ever pretended their camera are high ISO kings.  They seem to optimize their cameras for the lower ISO range, great dynamic range at lower ISO, and for color retention as throughout that ISO range.  It's one of those give and take situations: you can't have everything, you can't have your cake and eat it too.

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Paul