Thin mount CPL filters work fine on the 17-40 with FF, without increased vignetting. I use Lee filters, which are fine, if I only use the single holder, but if I ever need to twist filters in different planes, I can't because the second holder that slides onto the front is visible in frame, even at 24mm. I actually find 17mm to wide for the type of shooting I do on FF and like you I find it soft, so mine is also destined for eBay or somewhere else, as I think I've used it once in the last 12 months and that was for a specific effect, making use of the extreme distortion.