No samples on line, I am afraid. Don't have this luxury and envy the many good EOS sites reviewed regularly. To answer your questions; the filter is thick / thin like any filter, but I combine it with a standar UV filter from B+W; would you think this could be the cause? Light fall off is not the case as NA used was mid range with the samples under discussion.
Stacking these filters is pointless. The UV filter does nothing the Polarizer doesn't already do. Never stack a UV filter with another filter. At times, you may want to stack a Polarizer with another coloured filter or ND grad etc, but leave the UV off.
As far as the vignetting issue is concerned; there are easy ways to check this, and taking pictures is not one of them (that's hard, time consuming and costs some money).
Get a cable release, or in this case the remote release; fire the shutter on B with the lens at f/2.8 and open the back (I forgot whether the EOS3 lets you fire the shutter with the back open). Hold the camera up pointing it at the sky or other bright, evenly lit source. Look through the back and see if, while looking through the corners of the film gate the light is cut off by the filter; ie, look through with and without the filter in place to see what it should look like without the filter and then look through with the filter on the lens to see whether the shape of the lens opening gets reduced by the filter. You can also hold the camera so that the back is pointing up at the sky, and look through the lens so that you can see the corners of the film gate, and then you can tell quickly whether the filter (or lens hood, or whatever) is cutting into the image path. Check this at infinity and at close focus, and with zooms, check at various focal lengths.
No film required, fully accurate even without access to a densitometer, and quickly done.
This technique is what you should also use with your 1.3 or 1.6 factor DSLR to find out whether you can use a narrower shade on you lens. Here, of course, you just look through the lens to see the corners of the viewfinder mask, as you can't open the film gate. Allow a (very small) amount for the fact that the viewfinder does not show 100% of the image. Using this I was able to put a rather tight rubber lens shade intended for a standard (?) focal length on my 16-35, have better shading and not carry the monster shade that comes with the lens. For the 28-135 I use a 72-67 reducing ring and another standard rubber 67mm shade, for the 50/1.4 a hood for a short tele, and for the 70-200 and 100-400 a long tubular shade that is way smaller than the shades that come with the lenses, and shades better. For the latter lenses this isn't such a big deal because the shades that come with them fit nicely in the reversed position in the case that they come in, but the smaller shades makes life a lot easier with the shorter lenses. And good lens shades are _always_ a good idea.
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