Obama didn't overperform in IL-12, he only got 2% more than Kerry did.
Compared to similar areas in the Mid-South he did. Many of those areas trended away from the Dems compared to 2004.
But isn't that a mirror image of Obama's overperformance in Illinois--a personal opposition that exceeds the partisan line?
My point is that without the connection between Obama and his home state, IL-12 would have had even lower performance for the Dems. The PVI there would probably not be D+2, but something closer to D+0.
But if someone other than Obama were running, the PVI might have been D+2 for other reasons, just as Missouri might have voted Democratic given the economic conditions of the time.