I had a 270 for quite a few years but gave it to my son. I shot several moose with it when I live in AK. I don't believe that using a 7x57 would have worked any better. The diameter of a 7mm is only .007" larger than a .277 bullet. If you think .007 makes any noticeable difference, imagine the difference .024 of the 30=06 would make.
I have shot moose with 270, 308, 30-06, 300 H&H and 338 and didn't see any real difference in the results.
I have heard quite a bit of O'Conner but haven't read much of his writing.