My RS1 is mounted around 5 inches right of center on my 123" diag due to the stud location of my ceiling. After I center the image on the screen the left side of the image is clearly smaller than the right. Is the small amount of horizontal lens shift responsible for this? Any thing I can do to fix?

Your projector is not lined up with the screen try moving the projector left or right a little and then move the image back with the lens shift

Which is to say, the front of the pj needs to be parallel to the screen.