I don't know, I think the Manning effect was more the leadership he produced and the respect he commanded. I'd say Siemian is playing better than either Manning or Osweiler did last year.
Siemian 2016 11 games started:
2,730 yards 7 INT 16 TD with a 89.7 QBR
Manning 2015 10 Games:
2,249 yards 17 INT 9 TD 67.9 QBR
So you are correct that the QBs are better statistically, but the production feels off. The 3rd down struggles are real head scratchers. It's going to be tough to get help on the O-Line moving forward.