Enel was a special circumstance. Where he lived and what Devil Fruit he ate were two important parts to just how powerful he was and appeared to be. His powers were best suited in Sky Island, one commonly noted fact was the fact that in Sky Island, rubber didn't exist and therefor things such as conductors and insulators weren't explored. Being that he lived in a place comprised mostly of cloud and water-cloud, his electric abilities were unmatched by just about anyone there. His power wouldn't have been nearly as potent on the Blue Sea, though still very formidable nonetheless. Because of the fact that he is electricity, he has the advantage of raw energy. In feats of god-like destructive force it doesn't really seem like anything can come even close besides fire and maybe magma.
Though I believe that type of power puts him leaps and bounds above most people, but I'm also a firm believer of the power is only as good as it's user. Certainly he had quite a bag of tricks as far as the uses of his power, and he was well rounded in the knowledge of electricity, but he didn't have the demeanor to really carry that devil fruit. Had you given that fruit to someone like Crocodile or Lucci especially, he would have put that fruit to far more use than Enel did himself and went about it much more intelligently.
While you can say that he would be on definitely Shichibukai level doesn't really account for too much being that one of the first major villains Luffy ever defeated shortly after his entry to the grand line was a Shichibukai. Then again, a Shichibukai could also be someone like Kuma or Mihawk. As far as being an Admiral is concerned however, he is far from the type of person who would ever stand a remote chance against an Admiral, to consider them in the same league is an insult to that title.
As for Mantra, there are two distinct differences between them that makes Mantra far inferior to Haki. Mantra is the ability to hear the voice of the person you're focused on. With that, you can hear in their head what they plan to do next, what they are plotting. So like Luffy, if they had stopped thinking of what they were doing or attacked at random, the user would be clueless as to what their moves were going to be. Haki however, shows you a premonition of what is about to happen next and you can either continue as you would or counter what you saw ahead of time. Whether they were thinking about it or not, you were to see where everything was in those premonitions and could rearrange yourself accordingly to dodge. It also gives you the ability to hit Devil Fruit users despite their intangibility.
So really, if he were to make it to the Blue Sea, he would definitely have a good run with what he had. He still doesn't hold a candle to any of the stronger people in the series, rubber or not.
