I've read various forums suggesting he is an Omega Level Mutant (I agree with others that this term maybe needs rewording or defining) however in my experience he has never been officially mentioned as Omega Level. Some feats that he has managed should put him at Omega Level. 

As far as I can see Magneto should be Omega Level as I think he is powerful enough to alter the environments on Earth by doing something with the Earth's Magnetic Field which in theory this could destroy the world. 

If someone could give some extracts/info on the limit of the his powers. It would be great

Cheers