I am no expert, but perhaps vignetting depends on the width of the front opening of the lens where as crop is the final image on the sensor. The width/focus of the beam will change in size from the front to the back of the lens. This may be true more so in an EF-S lens, OTOH if it were an EF lens, I'd be more concerned.
I get vignetting below 20mm on my Cokin P, on a 24-105 F4.
Is your holder 1 or 2 filter slotted? I know guys who have hacked off the last (protruding) slot making it only a single slot holder, but that gives them extra width. One guys said he could do 17mm after that... but I forget exactly. Perhaps a search of the forums will lead to some answers.