There's a definate pattern in comics, especially the X comics, where when good guys become bad their powers increase without explanation and when bad guys become good their powers decrease without explanation. It's not like Hal Jordan becoming Parallax and becoming more powerful trough absorbing the Guardians' energy and chronic energy. It just happens.
From a story perpective it makes sense. If Magneto is just as powerful as an X Man as he is when fighting all of the X Men at once he can do most of the fighting for them. That's why Magneto seemed so much weaker in Uncanny 200, the final part of his first reformation, than in Uncanny 150, when he didn't care very much that he was facing all of the X Men. He has to be weaker to give others a chance.
On the other hand it's annoying that we know that these characters are capable of and that they should be able to get out of certain situations or resolve others with ease that they don't for this reason.
Case in point: Whenever Magik is depicted as having gone off the deep end or evil completely in the alternate universes she is depicted as being able to fight off all the X-Men at once, laughing all the way. Nothing short of the Pheonix, and even then only when fully powered, has been able to beat her in those situations. And when the Beyonder resurected the New Mutants she went toe to toe with Thor and two other Avengers, forcing all three of them to retreat. And this is all on Earth, not in Limbo. But while on the New Mutants she almost never shows a hint of that level of power. It's strange to see someone who showed the ability to beat Thor have trouble with random mutant #42.
One explanation is that while being a good guy you're constantly dirstracted and worried about your teammates and any civillains but when a bad guy all you have to worry about is yourself so you're free to be fully unleashed, but that doesn't fully explain why there's so much of a difference between the powel leverls of certain characters when they switch rolls.