I remember reading Howard Schultz's book "Onward" about his tenure as CEO of Starbucks, and how there was a significant return in their investments in employee education / benefits / stock incentives / better service, despite pushback from Wall Street.

Idk if you'd consider the two the same scale (the market cap for Wal-Mart is probably 3x that of Starbucks if I recall) so maybe that's _some_ evidence?

Though I agree evaluating/comparing the ROI on the two is probably not the cleanest analysis in the world.