It seems that by every proportional metric imaginable Costco which pays a higher wage is able to beat Sam's Club (the Walmart unit with which it directly competes). Costco has way fewer employees but moves almost the same volume. Costco makes more profit per employee. Costco makes more profit per square foot of store space.
Higher wages simply mean more efficient and dedicated workers and that really translates into good business. A lot of us could have told you this a long time ago but it seems that Wall Street still doesn't understand it, despite the numbers.