> The US saw 60% real wage growth from 1860-1890 with no empire whatsoever

I understand what you mean. But also: https://en.wikipedia.org/wiki/Manifest_destiny

You are right that common people in Britain didn't get as much out of Pax Brittanica as America's did during its own period of expansion.

Quite a different situation. An empire is when you go to a populated place and extract wealth from the people who live there. That’s not what manifest destiny was. America expanded into land that was sparsely populated by natives americans and mexico who had no wealth to extract.