The earlier two look almost normal in comparison ;). I think you've pretty much nailed it, though Grath3's pose makes it look like he's puffed - LOL. The only other thing I can think of is widening his face a little. I seem to recall in classic Life Drawing that there are a set of ratios that define the human face, so the more you alter these ratios, the less 'human-optimum' the face becomes, which is kind of the effect we're going for, I believe.
Yes, I agree with ya on that.
Oh, forgot to mention that yes, he does look a bit tired, LOL...I wanted to pose him in a different way than just standing up holding a big gun. Actually not a big fan of the Grath in that they can only be taken out by a head shot. Even though there are no more than one per side in a given fight, it still seems to make them too powerful. Not sure if I will ever use them or not.