Canon have never managed to get a single sensor into the top 10 of DxO Mark.
Hmmmm. Let's have a look: the top ranking Canon is the 1Ds3 from 20-Aug-2007. It currently ranks "only" 13th place on DxOMarks list. But all higher ranking cameras are newer. They date..
- from 2012 (2x: D800, D4)
- from 2011 (2x: IQ180, NEX7)
- from 2010 (2x: 645D, K5)
- from 2009 (2x: P40+, D3s
- from 2008 (3x: P65+, D3x
That puts the 1Ds3 in the 1st or 2nd position when it was introduced in 2007. Note also that five of these 12 cameras are in a much higher price class and some of the others are very recent. So although Nikon did overtake Canon in sensor noise, the 1Ds3 and 5D2 were state of the art in the year they were introduced.
I'd personally be surprised if it was better than the D700.
The D700 has similar scores to the 1Ds3 and 5D2 (80,80,79). So that would imply that Canon hasn't improved at all since the 1Ds3 (2007) and 5D2 (2008). Initial reviews show significant improvements. So this can't be right.
Based on reviews so far, Canon may just have less noise at high ISO than the D800. Not sure about the other 2 measurements though. So although the D800 is great and is likely to stay in the lead, the Japan rumor is undoubtedly just a rumor. My guess would be a score somewhere in the range between D3x|D4 (88|89) and D800 (95). After all, Canon probably used the D3x (2008) as a benchmark.