If you go with circular filters, you can stack ~9mm of filters on a 16-35 II with no mechanical vignetting (you'll get a bit less than a stop more optical vignetting in the corners). So, you could stack a B+W 10-stop ND with a B+W XS-Pro CPL, for example. But the Lee system allows you to adjust your polarization before slipping in the ND filter.
Have you tried something like this combination. I've personally found that stacking a CPL and a ND8 makes composing a shot too difficult for me because of focusing issues and/or adjust the polarization to my liking is extremely difficult. How do you get around it when more than 10 stops of light are being cut in front of the lens? how do you manage with the dark viewfinder?
I've repeatedly got vignetting while stacking non-slim NDX and CPLs ... but I was using the standard Hoya filters, not B+W. Probably the quality of the filters let me down more than anything else
It's no problem with an ND8 and a CPL, but an ND8 is only 3 stops and the Live View exposure simulation is certainly fine for the 4.5-5 stops of that plus the CPL. But, I suspect you meant a stronger ND like an ND400 darker. In that case, with CPL and a very bright scene, Live View can still usually handle it if you bump the ISO to focus and adjust polarization. But it's not optimal, and doesn't work if the scene isn't as bright.
Really, the best approach is usually to focus, adjust polarization and meter before putting on the 10-stop ND. Since you don't want the camera to try to focus and adjust exposure after mounting the ND, you need to be using back-button AF or switch the lens to MF after focusing (or just use a TS-E lens or Zeiss prime
), and be in M mode. With the TS-E 24mm, I often just hold the ND (easier with the Big Stopper, doable with the round 10-stop). Since I have a Slim CPL (no front threads), that's my only option, but with front threads you could hold the edge of the CPL to prevent the filter form rotating, and screw the ND in a little bit.
Also, since this thread is about the 16-35L II, I should point that using a CPL with a UWA lens will result in uneven polarization, particularly noticeable in a blue sky.