I went with 2:1 Centennial/Simcoe only because Shaun O'Sullivan said in an interview that it was their standard dry-hop mix that they used on most of their beers. It's possible that Bitter American gets a different mix, though.
I'm the guy that worked on that recipe. My second attempt at a Bitter American clone just finished and I've put up my tasting notes: Tasting: The Sociable (Batch #31) | Hopwise Brewery
It's still not an exact clone, but it's closer. I tweaked the recipe further, should anyone want to give it...