And to get back to my original point, 40D images are superior to 20D images, even without stooping to extreme pixel peeping. At least with my copy of each camera.
Pity you've not been able to demonstrate it with those sample RAW images you've taken the trouble to post .
The clear resolution advantage of the 40D at the edges of the frame in the ISO 100 shot is a bit of a mystery. Since I'd downloaded all your RAW images initially but hadn't taken the time to examine the other ISOs, I decided I'd check out if this noticeable softness at the edges of the 20D shot at ISO 100 is apparent in the other shots.
Surprise! Surprise! It isn't.
In fact, at higher ISOs the resolution at the edges is even more closely matched than it is at the centre. This could be explained by the fact that 35mm lenses, even on cropped format cameras, are not as sharp at the edges as they are in the centre. Any subtle differences in resolution between the 2 sensors will be more apparent with a sharper lens and almost all lenses are sharper in the centre.
I believe your ISO 100 20D shot is therefore an anomaly and should not be used for resolution comparisons.
In the comparison crops below, at 100% and 200%, I interpolated the 20D shots to the same file size as the 40D shots but did not apply any additional sharpening. Both images have had only the ACR default sharpening of 25 applied during conversion.
I also compared shadow noise in the darkest part of the images by greatly lightening both images with 'levels'. As you can see, both images are as close as matters regarding shadow noise. Conversion settings were: zero shadows and zero contrast with everything else at default.
Now I know that you didn't apply the in-camera noise reduction for theses 40D shots. If you had, then shadow noise would have been marginally improved. However, I would suggest that such improvement is negated by the fact that the 20D has a slightly higher ISO range than the 40D.
You are probably aware that Canon DSLRs have in the past typically understated the ISO values, but this has changed with the 40D. The true ISO range of the 20D is ISO 125 to ISO 4000, compared with actual ISO 100 to 3200 for the 40D. This is reflected in the consistently longer exposure values in all the 40D shots which you've had to apply to avoid underexposure.
Now I'm going to make what I think is a very valid point. If one is going to pixel peep, then do it properly which means taking all pixel-peeping factors into account.
If the 40D has marginally less shadow noise, when noise reduction is turned on, it's a consequence of a slightly longer exposure (more photons impinging upon the sensor). A slightly longer exposure is likely to contribute to greater camera shake and consequently slightly less sharp images which in some circumstances will also negate the slight resolution advantage of the 40D.
For example, if getting clean, sharp images is the goal, then the purpose of increasing the ISO setting is to get either a hand-holdable shutter speed to reduce camera shake, or an adequate shutter speed to freeze subject movement.
At whatever ISO setting you choose, the 40D is at a slight disadvantage here, compared with the 20D. The only circumstances I can think of where it's not at a disadvantage is the taking of stationary subjects on a tripod, as in your RAW samples.
If I need ISO 800 on the 40D, for example, in order to get a hand-holdable shutter speed of 1/25th, I can use 1/30th on the 20D at ISO 800 and the same aperture.
Now the differences in sharpness that on average would result from these differences in shutter speed (and it would have to be an average due to the variable nature of camera shake) would probably be of pixel-peeping proportions, just as the increase in resolution due to the extra 2 megapixels of the 40D is of pixel-peeping proportions. One negates the other, so on balance it ceases to be an issue or an advantage for the 40D, in most circumstances.
In conclusion, I can only repeat that any 'objective' image quality improvements of the 40D are only discernible at the pixel-peeping level, assuming the focussing is not an issue as it appears to be in your ISO 100 shot with the 20D.
Nevertheless, accurate focussing can be very important when using a shallow DoF. A more revealing comparison between these two cameras would be a series of images taken with a high quality lens, say the 85/1.2 at f2.8, focussing on the eyelashes of a model or the pet cat. If the 40D produces consistently sharper images at the point of focus due to more accurate and/or faster focussing, then that's a very worthwhile improvement.EDGE COMPARISONS
[attachment=3900:attachment] [attachment=3901:attachment]CENTRE COMPARISONS
[attachment=3902:attachment] [attachment=3903:attachment]SHADOW NOISE