F9 1.0's payload to LEO was 11.5 tons, vs. (up to) 6.8 tons for Delta II.

Yes, D9 was designed around boosters. This means D9 is a different kind of design than F9. I'm contending that a more F9-like design, like Saturn 1B, could have evolved into something like the F9 over time. D-II could not have.