I don't think we know Luffy's true post time-skip strength yet. In all his major fights so far there has been some factor that has added ambiguity to his strength. Arlong kept eating those power pills whenever he was struggling against Luffy, Luffy struggled *a bit* against Caesar because he could remove all the oxygen from an area, etc. Whenever Luffy has fought someone (since the time skip) without some incredibly unfair advantage, he completely dominated them in every conceivable way. In one case with his hands literally tied behind his back. Personally, I'm not convinced Luffy wouldn't be able to defeat an admiral. In the past all the Admirals have seemed insurmountably stronger than Luffy because he didn't have Haki yet... obviously that's changed.