You raise a good question. I think the DXOMark website can partly answer such questions, if you use the 'screen' mode which compares performance at the pixel level.
It looks as thought the smaller 7D pixel is significantly worse
than the larger 6D pixel in all major parameters, such as SNR, DR, Tonal Range and Color Sensitivity.
At high ISOs of 1600 and above, the DR of the 6D pixel is very much better. At ISO 800 it's still significantly better, being close to a whole stop better. A 1/2 stop of improved DR is considered to be noticeable in practice.
Even SNR at 18%, for the 6D pixel, is well over one stop better than the 7D pixel at ISO 800, and Color Sensitivity is 2.3 bits better (a 1 bit difference being of noticeable significance).
I remember years ago when I upgraded from the 6mp Canon D60 to the 8mp 20D, the increase in pixel count was irrelevant. What was really noticeable was the lower noise and improved color at high ISOs. However, the 7D has over double the pixel count of the 30D (or the 6D cropped to the 30D format), so I expect that at all ISOs the 7D would provide higher resolution but noisier images.
Whether or not such higher resolution would be significant after noise reduction has been applied to the 7D image, at large print sizes, is another question which can only be answered by someone doing comparison tests. However, the DXOMark comparisons at the link below indicate that the 7D image, when downsampled to the same size as the 30D image, has approximately the same noise, except for DR at low ISOs which is almost a whole stop better at ISO 100, and 1/2 a stop better at ISO 800. But the improvement would be less if one were to downsample the 7D image to a cropped 6D imagehttp://www.dxomark.com/index.php/Cameras/Compare-Camera-Sensors/Compare-cameras-side-by-side/(appareil1)/836%7C0/(brand)/Canon/(appareil2)/619%7C0/(brand2)/Canon/(appareil3)/179%7C0/(brand3)/Canon