I saw this interesting question and answer segment on the uncanny x-men writer's site:

Why does Magik experience a tremendous loss of power on earth? she is basically a hell lord level character in Limbo. Does she just hold back? She does not seem to use her time manipulating powers as well.

You've answered your own question. Generally speaking, Hell-lords have close to absolute power *in their own dimension*.

KieronGillen responded yesterday

How powerful is she on earth? does she still operate on a lower herald level? What are her feats on earth and outside limbo? she still seems to have a lot of feats....does she go from hell lord level to low herald? or even lower?