"Shoulder Devil (Su): At 8th level, an Asmodean advocate can choose an imp familiar as though she had the Improved Familiar feat."
In this case, the intent seems to be for it to work the same as the feat.
The vigilante magical girl can apparently do it though.
"At 3rd level, the magical child's familiar reveals another aspect of its form, and its vigilante identity changes into a creature on the Improved Familiar list that would be available to a 3rd-level spellcaster (the animal guide's social identity always remains as the original normal animal)."
And the wasp familiar can almost do it, but the result isn't quite an imp.