I can play most of the RB2 and RB3 setlist on Expert Guitar/Bass with about an average of 85-100% depending on difficulty. I can't play Pro Guitar/Bass for crap, so I'm obviously not Expert. I can play most of the RB2 and RB3 setlist on Expert Drums as well, at about 80-100% depending on difficulty. The hardest song I've completed on Expert Drums is probably Aesthetics of Hate. I'm not exactly a super drum player like some of those on youtube, I've only taken drum lessons for about 3 1/2 years now.
In terms of DLC, with the RB3 set list I know it said I had about 160 songs, so about 80-100 DLC songs, although I'm definitely getting more.
I had Green Day: Rock Band Plus so I imported the whole setlist. I have NOT imported the original Rock Band setlist.