If I understand your description correctly, I would try building out the wall with a couple coats of joint compound feathered in. Not going to get a full 3/8, but enough so when the back splash is installed, only you wil know you're not totally square.... You are more concerned with where the backsplash meets the wall anyway, and you'll be siliconing the top of the backsplash. It will be more important to keep the reveal uniform on front.
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