I've noticed the same thing in comparing G1 photos to those from my Epson R-D 1, and remarked on it in another thread.
Originally Posted by petermcwerner
My observation is that basically, the G1 has good noise performance even at high ISOs as long as you expose correctly, but you dare not underexpose with it at all.
By comparison, it's usually no problem to rescue one or sometimes even two stops of underexposure in raw files shot with the R-D 1. I assume that the difference is mostly because the R-D 1's sensor, even though an older design, is only trying to pack six megapixels onto an APS-C-size sensor. The G1, with twice as many pixels on a smaller sensor, poses a greater challenge in electrical isolation, as Jonas noted. Your M8, with 10 megapixels on a larger-than-C-size sensor, also represents an easier-to-solve noise problem.
Another thing that occurred to me: In comparing the G1's noise results to those of other cameras, you have to keep in mind that the G1's ISO-sensitivity settings are on the "hot" side, as you can see by the graphs on the DxOMark website. I had been comparing noise in photos made with the G1 set to ISO 1600 against photos made with my Nikon D300 at the same ISO setting, and noted that the G1 photos seemed noisier.
When I compared the graphs on DxOMark, though, I realized that's partially because the G1 is almost one full stop more sensitive at any given ISO setting than the D300 is. In other words, if I had compared actual exposures rather than ISO settings, I would have found that the Nikon has to be set at almost EI 3200 to deliver the same exposure level that the G1 delivers at 1600! When I compared the noise performance of ISO-3200 shots with the Nikon against ISO-1600 shots with the G1, I found their results to be more similar.
I don't know if this will work for you, but I've been experimenting with the following procedure for getting the best exposures from the G1:
When I start photographing in a given lighting situation, I choose an ISO setting that gives me roughly the range of f/stop and shutter speed I need, and use the AEL to lock in a representative midrange exposure.
Then, as I compose pictures, I adjust the exposure compensation dial while watching the in-finder histogram. I adjust the dial to move the histogram toward the right until the lightest highlights just begin to clip. This assures that I'm giving the most exposure the scene can handle
It sounds more complicated than it is; once the exposure is locked, it's very easy to check the histogram and turn the front dial a bit if needed. Incidentally, this exposes a bit of clever design in the G1: turning the front dial to the right (from the photographer's viewpoint) moves the histogram to the right, and turning the dial to the left moves the histogram to the left.
Sure, I'd rather have the G1's 12 megapixels and small size AND the tonal range and noise performance of a larger, lower-pixel-count sensor. But this is a new world and, as is the case with any new category of camera, it takes some experimenting to figure out how to get the best out of it.
Incidentally, one of the claims made for the GH1 (that's what they're calling the newly-announced video version, right?) was an improved sensor with better low-light performance. It'll be interesting to see how true that is and whether it has significant still-photography benefits. For now, though, I'm managing with the current version. Just remember: don't underexpose!