It's a 2 stop ND. I used it a bit when I tried the 16 on the G1 years ago. I still have the 15mm Hologon for Leica, with its ND filter etc. A wonderful lens for its time, and soooo much smaller than the 15 I got for the Nikon about the same time.While certainly not youthful, I still lust after the Hologon. I think it is a 4 stop ND filter. Not for the faint of heart.
And, yes, you can use it on the A7/7R (with the expected color issues).
See: zeissimages.com Forums - CornerFix Profile for the NEX-7 + Hologon 8/16mm Contax G mount
Henning