Should I get a slim filter?

Now that I have it I'm not sure if I needed it. I figure when I got my 16-35mm finally on a full frame body I would run into some vignetting issues. Without testing out my theory I went ahead and ordered the filter when I had a well worn non slim filter sitting on my lens the whole time. I had been taking it off when I was shooting with it and then screwing it back on after. The day the new filter came in is when I ran my test, I took a shot with the non slim filter and then took another shot with the filter off. I only shot with it on the 16mm end and I don't see a difference. I'm at a lost at why I paid extra for the slim at this point. On the site it did say that the slim was made for the ultra wide lenses but whats more wider than than a 16-35? You can't filter the 15mm fisheye cause it bubbles out. Maybe the 14mm might be just wide enough for the slim to come into play. Maybe the 10-22 on a crop body but I doubt that.

The issue now is that with a slim filter there is no front tread so I can't use the more secure snap in caps. I'm stuck with the tension condom like caps. Already dropped the cap once and had a car run it over. I have to be extra careful with the filter now. Lesson learned.