Thinking about starting Alex Smith over Stafford
I wouldn't do that.

I posted the same 2011 stats for good QBs against SF last year and it seems like people are ignoring my post above. SF in 2011, regular season and post season, was not a great pass defense, especially against good QBs. If anyone benches Stafford for Alex Smith, they get what they deserve. Benching Stafford for Ryan is different as I am extremely high on him regardless.
I don't think this is about the quality of the niners pass D. We can debate whether or not those games accurately reflect a suspect pass d given that Brees and Manning both were around 60 attempts and Rogers's 44 would have been his second most in 2011. The reality is, I expect DET to pass something along the lines of 50 times and I expect a similar gameplan from the niners, namely taking away deep passing by having Goldson play deep while allowing a bit more room underneath and relying on the superior tackling of the LBs to keep any short passes as short gains. Stafford is a very safe start this week, though I would be concerned about him finishing the game. Niners had Rodgers dead to rights on several plays that he escaped thanks to his scrambling and Stafford isn't close to the caliber of Rodgers when it comes to escaping. Calvin is a must start every week, but I'm not expecting a big game from him. I'd actually be okay starting Smith in PPR leagues, expecting 6-8 catches out of the backfield without much yardage, but almost a definite sit in non-PPR. Pettitgrew seems like an obvious one as well.