Clinically insane can be a lot of fun. Although, not always.
That said, wouldn't him being completely off his rocker make him the most powerful ninja ever? Or, at least, make those who subscribe to the theory that the more powerful the ninja, the less mentally stable they are.
Now, let us be fair, Gaara at that time was mentally unstable in a very powerful way, making it more of a fair fight. And Lee was doing a very good job of beating Gaara, until he got his leg caught in the sand.
Gai... He probably could have if he used shadow clones. Don't know if he can though. There is only so much ground a person can cover while fighting. Even if you are one-shotting people left and right.