Actually, Marc, what I'm going on is the phrase from the review "I would estimate that between the lack of fast wide lenses and the abysmal high ISO performance, the M8 is 4 to 5 stops behind both Canon and Nikon in low-lighting shooting" and where he points out earlier that this is partially due to the slow wide-angle lenses.
Now, I'm going on secondhand information here, because I don't own an M8. But getting barely acceptable images at 640 ASA doesn't sound very good, given the 1.3x crop sensor. Compare to the 1D Mk III's high ISO performance. Which I suspect just goes down to Canon and Nikon having a lot more money to blow on camera-integral and raw-converter noise reduction than Leica.
But, yes, there's a better chance that with a high-quality near-diffraction-limited lens and the bigger sensor area and the lack of extra glass between the lens and the sensor, you could squeeze more optical performance out of it in a smaller package than the 5D in sunny situations.
