In my honest opinion, I doubt that Stan Winston had anything to do with the SS piece (with the exception of his name pinned to it), I don't think he even saw it, and actually approved it..
That is my own opinion formed by talking to some people trough time, and even more, by reading "Winston Effect" and watching some documentaries (mostly behind the scenes)..
You probably know, but Winston was a perfectionist, and SS skull simply has too many flaws that I don't think Winston would approve of..
Again, it's my opinion, I respect others, and I'm well aware that I'm in minority..
Icons skull would be "too round" while HCG, I believe is second generation recast of the actual screen used one!
SideShow is obviously NOT screen accurate, but "idealized" version of Terminator skull..It has some details softened, and overall a bit changed shape, also as mentioned many times before:
it has oversized CPU cover......it has oversized chip......most of the skulls are cross eyed....battle damage paint is awful....and teeth, no words for that.
And after all that, it's impressive skull and I wouldn't mind having it.....it's just matter of taste..