Well it might last 4-5x longer if you run safari in the background, since that is what you tested. But wouldn't the real test be running both browsers as the active window and comparing those results?
One thing that Safari is incredibly good at (and its' engineers are proud of) is the ability to reduce resource consumption for non-active tabs and windows to nearly zero. This is really good when running on battery because it is a shame to dedicate resources to things that aren't even visible on screen. But this could make the results far more dramatic through your test.
I would hypothesize that Safari still wins your performance test (if you were to re-test using them both as "active" windows), but probably not as dramatically as in the inactive window test.