I saw this vid on youtube, where this guy was holding the IM bust and he put his hand on IM's faceplate, and it looked fine to me. By comparing the guy's hand, and IM's face, it looks fine. And their head sizes were about the same too.
A guy on the Replica Props Forum,Bats,has stated that the bust was shrunk down 1%,purposely,to prevent recasters from selling it a s a proper overhead mask.
This guy knows his stuff,has contacts,and spent a whole day with the Mark 3 flightsuit prop going over it.He s very impressed with the Sideshow bust and says that the colours are as near a perfect match as dammit.