actually there are a few AMG (engined) S-classes, but no W221 AMGs, just W126s W140s & W220s.
this COULD be a real AMG except it cant' tell from this shot. would you happen to have a front picture?i can tell from teh front bumper if it is a real AMG engined car or a simple AMG Appearnace package.
from the rear i'd have to say it possibly IS a real AMG though as the kit is original (As in a real MB part)