To be clear, the "uneven effect" issue applies to wide angle view of the sky, where the difference in polarized light from different angles of the sky becomes obvious and disturbing. For other (non-sky) uses of a polarizer with a wide angle, this isn't an issue.
I haven't used the Tokina 11-16. However, with the Canon 10-22 and a standard 82 mm B+W F-pro and 77-82 mm step-up ring I got bothersome corner vignetting only at 10 mm; pretty much gone at 11 mm. Now that B+W has come out with the XS line, I can use a 77 mm version directly with virtually no vignetting. However, that may not apply to the 11-16; the risk of vignetting depends on aspects of lens design other than just the focal length.
I have used the Hoya Pro 1 Digital slim CPL with the Tokina 11-16 with no vignetting at 11mm. This CPL works great for waterfall and landscape work and the Tokina 11-16 is a great UWA lens. The only issue I have with the Hoya CPL is that it's hard to clean until I purchased a LensPen for filters. Great little tool to have in your shirt pocket.
Ya, I had the Hoya slims but they got dirty real quickly and I felt they did not thread on very well either. Went with the 77-82 + 82mm filter for that reason- way cheaper than an XS-Pro filter and easily does the job.
I have that lens and use both B&W and Marumi CPLs. Vignetting is not going to be your problem so much as uneven polarizing effects, particularly if your shot contains a lot of open blue sky. Just one of the realities of using a CPL with any ultra wide angle. It's a hard thing to predict, but as a rule I try not to go under 21mm when I use a CPL. While I love the contrasty effect of a good CPL, those "blotchy" skys generally tend to be unfixable. Just the same, screw it on, take the shot, see what comes out. Electrons are the only thing you're wasting.
