I've got B+W MRC filters on almost all my lenses (bought before the Nano coat came out, new lenses will get a Nano). I have a mix of XS-Pro and the standard F-Pro mounts, depending on the lens.
The EF-S 17-55mm is actually pretty resistant to filter-induced vignetting:
Basically, you can stack a standard CPL onto an XS-Pro or a slim/Nano CPL onto an F-Pro and still not get increased vignetting.
Personally, I had an XS-Pro on my 17-55mm, bought before I did the above tests.
As for a VariND filter, personally I prefer solid ND filters, and I find that a 3-stop and a 10-stop give me the flexibility I need. In my case, that's partly because I often want long exposures with wide/ultrawide lenses. Starting at about 24mm on FF (15mm on APS-C), uneven polarization occurs - you see this with a CPL on wide/ultrawide lenses as banding in the sky. A variable ND filter is essentially a pair of stacked polarizers (one circular, one linear), and at wide/ultrawide angles, the uneven polarization manifests as a 'Maltese cross' artifact - a dark 'X' through the frame that gets more prominent as the AoV gets wider and/or the darkening of the VariND is increased (at the darkest end, you start to see the artifact even into the normal focal length range).
But...if I were going to get a variable ND filter, I'd likely be choosing between the Tiffen one and the relatively new Schneider Optics
version (Schneider is the parent company of B+W).