The one thing with any wide angle, zoom or prime, is that the closer you can between the last element and the sensor the better. I'm not sure how the Pentax works (i'll look for images once i've posted this), but if i were a lens designer i'd be putting the filter between the last few elements, whether the distance needed for that makes the design impossible I don't know.
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But also, what I do know is that using a CPL on 15mm on APS-C (24mm on FF), you really can notice the blue banding in the sky. Using a CPL on 12mm on FF would make a very very uneven sky...