Hi David,
I think the white is good to go,shading and filth are well done.
I kind of agree with Markus about the lips, but figured it was too far along that you wouldn't want to do anything about it. I also agree that replacing the head and helmet would probably be the easiest route to go, instead if converting this guy again. Unfortunately shouting heads aren't that easy to come by. The one shouting figure from Verlinden that immediately pops to mind looks totally demented, with bulging eyes, and he just basically looks weird. I have that figure and almost completely re-sculpted the face on it to get it acceptable. With few individual heads available out there any more, finding a better replacement isn't real easy. I must have 40-50 120mm heads, and only a few could even look like they are talking, all others have closed mouths. Perhaps someone out there has a shouting head that fits the bill and could send it your way, if you want to change it.
Cheers
Jason