Joker was far stronger than him before TS so I don't even get how exactly he can surpass him in just two years without actually having a DF. Joker is the strongest pirate at Dressrosa followed by Luffy. Burgess has nothing but hype which was pulled out of nowhere.
I won't say it comes out of nowhere. Burgess is the only one Sabo had some concern about before engaging himself in battle, battle in which he had some difficulties against him, to the point he was forced to break the floor as a distraction to steal the fruit instead. Koala even seems to fear him more than Doflamingo even telling that Luffy's life was in danger to the point of calling Sabo for help. Curiously as well, he is one of the only 3 in the island who doesn't seem to be worried about birdcage(the others being Fujitora and Sabo)
About the difference in strength pre and post-timeskip i won't say it's a fair comparison either. Doflamingo is a stepping stone and static character when Burgess(and the rest of Blackbeard pirates) are in constant evolution since they represent the final enemy of the strawhats in this manga, so they'd have to progress more and more during the saga
I'd also find it strange personally if Luffy was already stronger than a end of serie character unless he gets a devil fruit boosting him significantly
