Yeah I highly doubt Imperiex's Outer shell is a metal that mags could manipulate.
And I don't know the extent to which Imperiex can resist mind control, but I hope we all remember what happened when Xavier tried to pull that trick on Galactus.
If Onslaught has sucessfully absorbed all of Franklins and Nate Grey's powers, then maybe he might have a chance.
But as it stands Imperiex 10/10.