What do you expect to see or hope to test for? At a previous employer I worked at where people weren't happy, I'm pretty sure half would've quit almost immediately. Admittedly I don't have data on that but I'd expect that outcome.
I think the idea is that financially secure people are more willing to speak their mind. Getting salary upfront means everyone has small scale “fuck you” money. The problem here is that most people don’t actually have a reason to care about their employer’s problems, so not speaking up is often just a strategy of least resistance.