105mm Circular Polarizer..? Or 4x4 polarizer?

[D-B-J]

Does anyone have experience with a plate filter system and polarizers? My current setup is a hack. I use a 77mm cpol, then stack the wide angle adapter and my LEE system in front. I get pretty severe vignetting up to 22mm... which sucks. I really can't decide if I want to get the 100mm square (4x4) filter, or spend the extra coin and get the 105mm cpol that screws on the front. Even with buying a Sigma cpol the front-rotating style would be around $250 and the square one would be $216. I'm worried about vignetting at 16mm with 2 plate filters and then the cpol in front, so if anyone has experience or a site to point to (I couldn't find much info) that'd be great!

Jake