I'll put my 25cents on the line that 7D II will not out perform 5D III in high ISO/low light.
OK, outperforming might be a stretch. But being quite close is not unrealistic ... at all.
If you compare Nikon's D7100 with the 5DII (not III) on DxO, you will see that they are very close in every measurement.
The 7DII needs to be only slightly better than the D7100 and it will be in 5DII territory wrt image quality - which in turn is quite close to the 5DIII.
And honestly, if the 7DII is not at least slightly better than the D7100 in image quality, Canon shouldn't bother with an update, thank you very much .