Luffy already defeated Kaido, so right now he is already slightly stronger or at worst equal to Akainu.
Then he will receive another PU in later arcs. At least, he will get another one PU in Elbaf arc, but the more the number of arcs in the future, the more PU he will get.
By the time Luffy meets Akainu, Akainu will get neg diffed. Unless it is handicap fight like Luffy limits himself not to use devil fruit, or closes his eye with cloth, wears seastone cuff, or tie his hands in his back, so that Akainu can push him past low difficulty.
But that won't happen anyway, no intense doing so fight, so at best, Luffy will meet Akainu briefly then think "I will Blueno this bitch" but then Yamao comes and offering herself to defeat the Red Bitch to avenge Ace.
Newflash: no matter she is above or below Sanji, still if she joins she will be the one whooping Akainu's ass. There are too many hints already that point to that.