I use both the LEE and Singh-Ray filters and personally I find them to be outstanding, the IQ does not go down when used with 1 filter and does not go down very much when you stack 2 filters! Vignetting is visible but acceptable. I use the 16-35mm L II too with LEE system it works great! but usually the ND filters are 4x6, only my big stopper (So rare to see one and hard to get!
) is 4x4 but they will work, one thing I find is that at 16mm there will be dark corners on FF bodies, stopping down to 18mm and it's minimum and at 20mm it is completely out of sight, I don't know if anyone else is experiencing the problem. Another problem is that they NEVER, seem to have these filters in stock and it took me 3 months to get that big stopper...just insane.