Katakuri is probably amongst the stronger Yonko Commanders such as Marco, who can fight on comparable ground to an Admiral even if at a disadvantage. Factor in Luffy's Haki training and the Straw Hats becoming stronger from overcoming death (which should certainly count for his fight with Katakuri) and I'd see it as plausible for current Luffy to either stalemate or beat an Admiral in terms of power. The problem for him to overcome is their hax as all of them have skills to checkmate him (Akainu can burn through rubber, Kizaru has the range and speed to casually spam lasers from afar, Fujitora's gravity hax is very versatile and if counting Aokiji he probably has the most to take down Luffy).
