After I asked about it in this thread, I printed a target and took a couple of test shots with and without the extension tube.
Here's my EF-S 17-55 directly on the camera, no PP, full image:
Here's a full-sized image of the intersection at the upper-right of the same:
Now with the tube mounted, again no PP, full image:
And a full-size image of the upper-right:
The bottom line appears to be that the lens has some CA, which is to be expected. It seems to measure about 4-5 pixels on the blue side. With the tube, that CA at least doubles, to 11-12 pixels.
These were exposed toward the right, but not overexposed anywhere: no blinkies on the camera display, and no blown highlights indicated in ACR.
I wasn't able to focus exactly the same (that's the idea of the tube), so there's a little difference in distance from center, but I wouldn't expect it to make this pronounced a difference.