Been a while since I saw an Enma topic.
I always argued for it being a training wheel, cause I thought that by mastering it, he would just get an even better grasp of his haki flow, basically becoming able to release that haki cloud without Enma's gimmick, which actually did not happen.
So now I would say it's definitely a power up. He had to get used to it, but it's still a power up. W/o it he wouldn't be able to release that haki cloud, which seemingly boosts his AP considerably.
I would note though, that he has to activate it or rather let it suck out his haki, so outside of that gimmick Shusui still is slightly better.
So if he somewhere in the future would be able to release that shit ton of haki by himself, Enma's gimmick wouldn't be needed anymore and Shusui would again become the "better sword", if Enma isn't a black blade by then.