Big Block Cylinder Heads "915 vs 906"

I always liked the 915s. I could get an easy 270 from them with 2.14/1.81 and they always made great power. But - my first choice is 346s. They had what I feel are a better exhaust port with the "identical" intake port to the 915/906 and the same work and parts would get 280+. I'm guessing the chamber shrouding would explain the difference but because I test as a final step for validation - I never experimented with that. The 906F would be a 2rd behind the 915s.