Worstgen
Cinera
I then shifted to thinking that most of the strength increase would come from the training to master Enma with Enma granting a moderate boost to AP.
Cinera
After some further rumination, I think that Enma is a substantial increase to Zoro's attack power. My reasons are as follows:
Cinera
Enma is the only sword to have damaged Kaido. This is despite Oden favouring a 2 sword style. Oden's other blade was the same grade as Enma, but did not have the feat. It seems doubtful that Oden could replicate wounding Kaido without Enma. Thus it seems that a substantial part of the power of the attack that injured Kaido came from Enma itself.
Cinera
We've seen swords that do increase attack power. One example is Shusui, which in addition to being durable, Zoro noted that it swallowed up the airslashes of his other 2 swords when he used his 108 pound canon. Even post timeskip, 1080 pound canon is one slash, while we see that Zoro's 720 pound canon is two slashes. Thus the AP increase from Shusui is still present.
Cinera
Zoro also commented on the sharpness and bloodthirstiness of the Sandai Kitetsu when he first got it. The Sandai Kitetsu was also noted to be a cursed sword.
Cinera
Enma is also a special sword (though not a cursed sword). Hitetsu commented that the reason Zoro found it easier to tame Enma was because of the Wado Ichimonji which seems to have acted as a moderating influence.
Cinera
I am not really sold on @Van's (I think it was him) "Holy Swords" theory, but it seems clear that there are other kinds of special swords behind the cursed swords and Wado Ichimonji/Enma seem to be ones (black blades are another axis of specialness entirely as a sword can be both black and cursed or black and holy (or whatever the other kind of special swords are).