In Mike Green's first full NHL season (06-07), he averaged .17 points per game, while receiving more Power Play TOI per game than Voynov, and while playing for a team that averaged 2.85 GPG. Green produced 1 point for every 90 minutes and 20 seconds of TOI.
In Voynov's first full NHL season (11-12), he averaged .37 points per game, while receiving slightly less Power Play TOI per game than Green, and while playing for a team that averaged only 2.07 GPG. Voynov produced 1 point for every 50 minutes ice time.
Does this mean that VV is as good, or better than Green offensively? No, because stats don't tell the whole story and can't account for all the variables. For instance, Voynov was slightly older, and had more time to develop in the AHL before coming to the NHL.
These stats do give a little bit of perspective however, and show that comparing the two isn't as ridiculous as Herby made it out to be.
When Voynov peaks, I think he'll be one of the top PMD's in the NHL. When Green "peaked" (assuming his best days are behind him), he was the most offensively productive defenseman in the NHL by a significant margin. I don't think Voynov will get to that level, but I do think he's capable of being in the top 5 or 10.