It's kind of funny, the Barry arguments sound really similar to what my VT friends say about Vick vs Young. "Well if Vick had Texas' OL, and was in a backfield with Jamaal Charles...."
FWIW I do think Barry has a much better argument over Emmitt than Vick has over Vince, but the thing is that the "well imagine if Player X had an amazing team around him" argument is a bit too easy, even though it is sometimes true.
It's also interesting that supposedly Mark Stepnoski, Erik Williams, Larry Allen,and Nate Newton made Emmitt Smith, even though Emmitt's best season came when Mark Stepnoski was playing for Houston and Erik Williams was still recovering from his car accident and was a shadow of his former self that year.