Given the S at the beginning of Stevenson, I don't think I'd necessarily notice if someone said it the other way, unless they put in a dramatic pause or two. And I'm actually not sure whether USians typically say "Louis" with an S at the end or not, because (a) the name is out of fashion, (b) all the older men I know with that name are called Lou, and (c) if they weren't called Lou they'd be called Louie, if you see what I mean! (E.g., my father-in-law's boyhood friend Louie.) I do recall being surprised to find out that my internet acquaintance Dr. Margot Louis pronounced her last name Lewis.