What exactly are you expecting to be different, performance wise? If the quality of the Sword is better, you can expect harder, heavier/lighter and sharper blades, which the weight would effect the speed behind the strikes/users movement, and the lethality of the attacks. It's not a tangible thing you can see.
Whatever Enma's case may be, it seems to directly be helping to strengthen Zoro's haki in some way. That effect for Zoro is permanent, so it's a powerup like internal destruction Haki for Luffy is a power up. These things aren't like the Raid Suit, where it only boosts Sanji while he's wearing it, which would constitute that as a temporary powerup. Any kind of new, stronger ability, or strength boost to an already existing ability, is a powerup.
In history of all OP power-ups, there has never been a power-up that nobody could tell what exactly it is.
Every single one can be pointed out directly in combat. The situation with Enma cannot because it is no PU...
Just like people said, the weights used for lifting werent called a PU, Hyogoro isnt called a PU and so on...
With Zoro's restrictions towards haki drain, Enma is left with the same amount of haki as the other 2 swords and thus, no different.
Since Kokutos have been hinted at during this arc, I suspect that's the true purpose of Enma.
To improve Zoro's haki manipulation to be able to create a Black Blade and until then, nothing tangible will be in front of us.
But there is nothing new, stronger, boosting for Zoro's existing abilites when Enma is restricted.
Only if he allows it to drain him completely it would be the case which he obviously isnt doing.
As long as Zoro controls the amount of haki released, Enma is the same as Wado Ichimonji and Sandai Kitetsu.
Take away RS from Sanji and Haki flow from Luffy and they lose abilities, they are no longer capable of what they were with them.
Take Enma away from Zoro and he is still capable of the same stuff.
That makes it obvious.