I have noted a number of posts stating that the internal width of 8, 9 and 10 speed chains are all identical.

Per both the Wippermann and KMC web sites this is not correct. 8 speed is 3/32" while both web sites list the internal width of their 9 and 10 speed chains as being 11/128. A small difference of about .008" or .2mm but they are narrower internally.

SRAM does not list internal width for their chains, nor do Campy or Shimano that I was able to find. I would presume though that they are the same as KMC and Wippermann for compatibility reasons. No reason for them to narrow their 9 and 10 speed chains internally if Shimano and Campy had not done so.

External pin widths are as follows:

11 speed Campy 5.5mm
10 speed 5.9mm
9 speed 6.6mm
8 speed 7.2mm