With an EF lens on an APS-C camera, you can put a thick stack of filters with no worry about vignetting.

A while back I tested the EF-S 10-22mm - you can stack an XS-Pro and F-Pro (or an XS-Pro and Slim CPL) with no mechanical vignetting. Two XS-Pro plus an F-Pro and you see the filter. Details here:


Filter stacking and vignetting at 10mm by Dr_Brain, on Flickr