You think Isakson will only win by eight?
He'll win by more, but Senate races /=/ presidential races. I mean, Ron Wyden and Patty Murray will significantly outperform Clinton, yet that doesn't mean that OR and WA aren't inelastic states at the presidential level. Isakson probably gets support from a few remaining White Democrats and Independents in the Atlanta area and will do better than Trump with minorities (especially Hispanics and Asians).
Then why is the senate race from 2014 the best example? I'm not saying Trump should be winning GA by 20, but he should hope to be on the other side of Romney's win margin (maybe 10 or so points ahead) rather than under it.