This doesn't make sense, if it's not something you can see then it doesn't "seem" like anything. Throughout the raid we haven't been given any indication of improvement, Haki related or not. All we really know is that he's been intentionally conservative and managing to stop Enma from bleeding him dry. That's not a situation that sounds like a power up because the alternative was a sword that he didn't have to worry about in that regard.
We've seen the hardest Luffy and Sanji could hit before this, and what they couldn't help but fall to in combat. The closest Zoro has come to defeat is running out of breath underwater, or sleep gas, or passing out after whipping someone's ass while nerfed. Is he gonna perform better than he did in those cases? Yes, but that's implied by the time that's passed. That has everything to do with being given time and getting in the appropriate mindset.
How does it not make sense when Zoro himself stated that by the time he is done mastering Enma, he will be much stronger? Do you think he gave up Shusui out of the kindness of his heart? Of course not. He gave up Shusui for Enma because Enma is going to bring him one step closer to accomplishing his dream by making him stronger.
I fail to understand how you guys can so adamantly argue against something that was written by Oda, and stated by Zoro using the logic "we can't see what it has boosted for him so therefore it's not a powerup." We only see what Oda wants us to see, but that doesn't detract from what he has told us.
Let me ask you, do you think that if Zoro was never given Enma, that he will be as strong by the end of this fight than he would be by getting Enma? If so, then what was the whole point of giving him Enma and telling us it's going to make him stronger? There would be no point. However strong Zoro ends up after this is strictly because of Enma, ergo Enma is a powerup.