Great work GML, I love the addition of the little winglets on the helm, it's a really nice touch.
I'm glad that you didn't take the 'easy' option of just green stuffing robes over everything, I think that you've produced something quite brilliant and only subtly different.
Having seen the size of the Primarch, I don't think I'm particularly fond of the size at all. I figured that he would be a head or 2 taller than a Custodes...but not that he would looks as though he was scaled for an entirely different game system! I think it's all a little bit silly. Then again, Magnus is the size of a tower block...