I know this thread is a couple years old, but can anyone tell me if you get vignetting at 16mm with the Lee holder setup with ONE slot, and the 105 ring with a circular polarizer.
Forgive me, but pulling slots off the holder is a tiny/piecey mess, so I pulled my 2 slot + CPL ring setup out, removed the adaptor ring that threads into the lens altogether and tried to hold the holder manually over the lens in a one-slot closer than it ought to be position. I didn't center it well and the result was inconclusive.
My guess is that it that 100% depends on the specific CPL you use, but for a garden variety Lee or B+W 105mm CPL, I think it will still vignette slightly at 16mm. Why do I think this? The CPL outer rail that sticks out beyond the CPL ring is as thick as both
slots. It's a solid 8mm or so (B+W Kaesemann 105mm) versus perhaps the 4mm per slot, so my guess is you'd be somewhere between my first and second scenarios in my original post.
However, if you used either a slim 105 CPL or B+W's step-ring version
(105 thread + much larger outer ring to fight vignetting), the situation would become more favorable for you. Neither are great options, as the former would undoubtedly be a bear to secure and remove, and the latter costs a mint.