I'm sure some creativity could probably help to get the shoulder slope more accurate, but any 1/6 figure, especially using real leather, is going to look off because the leather can only get so thin, and it's still going to be too thick and look different from Arnold in the actual coat.
I will say, the overall tailoring on the custom jacket surpasses the work on any HT jacket. HT always struggles with how it wraps around the neck and the opening flows down his torso. When I had the original MMS117, I had to use a paperclip under the collar and lapels to get the jacket to close around the figure to look right. And it's not a matter of holding HT to some accuracy standard of nitpicking mm and angles, it's just that the way in which their tailoring is off is not only inaccurate, but it makes the jacket look more like a "doll" coat. The custom jacket looks like a real jacket on a real person.