If Broun or Gingrey win, this race would easily shift to Likely Dem, and Michelle Nunn would probably end up a shoo-in
As much as I'd love to think that, I don't think that will ever be a reality. Georgia could be the Indiana of 2012, and Joe Donnelly only won by five-ish points. Georgia's too red for any Dem to be a "shoo-in."
You might cite McCaskill's 16 point win in 2012 Missouri, but I think it's slightly different than 2014 Georgia because they had an incumbent running in a state where it's actually possible for Democrats to win statewide. Unless you're Evan Bayh in Indiana, no Democrats typically win statewide there or GA.
I could also see Georgia as more receptive to a Libertarian candidate, but that will only work to make the path to 50%+1 for Nunn more difficult. If it goes to a runoff, she's toast.