I think it's also important to reconsider that every fight, we have been shown from the strawhats yet, was either a one-hit-win or at a clear disadvantage. Imagine Luffy's fight with Hody on an Island. Until he learned Armament Haki, Luffy was always at a disadvantage against blades. Imagine him fighting Mr. 1 and Zorro fighting Enel.
Oda always finds ways to give every strawhat their best match up when it comes to important fights, which kinda overthrows the power-balance in theory. When it comes to Luffy, he should be stronger than any Vice-Admiral and maybe even stronger than most Yonkou-officers atm. He will still be inferior to Admirals and Yonkou by himself. And it would be awfully boring it he weren't.
Luffy is far stronger than Zorro, if it would be possible to measure their power on a scale. Otherwise Zorro would be the Captain. To me it's that simple. And Zorro being overpowered? Please show me a fight with an equal opponent. :P
I'm not a fan of discussions if Luffy would be stronger than Hancock, since they won't fight in the manga. It's clearly not intended and therefore could be kinda ignored.