In Enies Lobby Luffy fought an opponent that was in the same position as Sanji's and Zoro's opponents, and was even a rival of Sanji's opponent, despite the big difference of douriki.
So if Luffy fights Akainu his opponent will be less equal to Sanji's and Zoro's opponents than in Enies Lobby, since Akainu is a fleet admiral.
You may have the impression that Akainu is not significantly stronger than the other admirals, but that's just because none of them went all out yet, nor exhibited some number to express their power levels yet, for you to be able to see with clarity how much stronger Akainu is than the other admirals, just like one couldn't know that Lucci was much stronger than Kaku just by judging how they were portrayed in Water Seven.
So if Luffy fights Akainu his opponent will be less equal to Sanji's and Zoro's opponents than in Enies Lobby, since Akainu is a fleet admiral.
You may have the impression that Akainu is not significantly stronger than the other admirals, but that's just because none of them went all out yet, nor exhibited some number to express their power levels yet, for you to be able to see with clarity how much stronger Akainu is than the other admirals, just like one couldn't know that Lucci was much stronger than Kaku just by judging how they were portrayed in Water Seven.