My test setup:
5d Mark II, tripod mounted, 18" away, EF 16-35mm f/4L IS, shooting at f/8, B+W 77mm XS-Pro 007 front filter, Lee wide-angle adaptor, Lee holder with two slots, 105mm ring, and a B+W Extra Wide KSM Circular Polarizer.
My results were the same; it vignettes ever so slightly at 19mm and is completely clear by 20mm. Removing just the 77mm XS-Pro 007, I have the same ever-so-slight vignetting at 17mm and it's clear at 18mm.
If you want the maximum usable range on your 16-35mm with the CPL attached, get the Extra Wide CPL from B+W and remove the protective front filer: vignette-free from 18-35mm. If you insist on keeping the protective front filter on, just understand that you're limiting yourself to 20-35mm.
As has been mentioned, you can always carry a second Lee holder with only one slot. I've got no vignetting at 16mm with B+W 007, wide angle adaptor, Lee holder with one slot, 105mm ring, and the B+W EW CPL. You'll just have to decide between long exposure and balanced lighting.