Posted by Wirehead Arts
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.