Do you or a friend or a local camera store have 2 filters of equivalent rim height that would fit your lens? Stack them and shoot something solidly light (makes seeing the vignetting easier) like a blue sky using manual exposure to get exposure correct. Open your camera to widest aperture and then take a series of exposures at various focal lengths at BOTH nearest and infinity focus. Download and check on your monitor. You can run them thru Auto Levels if you really want to be exacting about it. Canon typically is pretty good about giving a wide berth for their lenses re filter diameters, even for 2 stacked. However, depending on the rim heights (you say non thin rims) even then its usually not feasible as you get to the real wide angle focal lengths. I'd expect no problems until you get down to low 20's (mm), at least on an APS-C camera.