When the crew came back to Sabaody, during the conversation between Rayleigh and Shakki, Rayleigh said that Luffy will still loose against an admiral but he will give them a good fight. And that's FI Luffy. We know what happened when Luffy first meet Kaido, which is post WCI Luffy.
I think this is clear enough to see the power level between Yonko and Admiral.