It's possible that the Intrepid Class Starship circa 2370 is about as powerful in a short fight as a Galaxy Class 2363, However by 2370, a Galaxy Class Refit with contemporary upgrades and seven years of walking the line would make the Intrepid look like the oversized shuttlecraft that it is.
Here's why Voyager seems so powerful, when Picard tried to run away in Q Who, he couldn't his ship was not fast enough or powerful enough to escape. They were going to cut the Enterprise up for parts and flush his crew into space.
Janeway's two true advantages are first that the the Borg assimilate civilizations not individuals and second that they had no more curiosity about humanity or the Federation. The Borg had nothing to prove and assimilating Voyager would render no technical or tactical advantage vs. the effort of tucking them into Collective.
Kinda like America and Puerto Rico.