I'd argue that workers who don't feel exploited are more productive and that more productive employees can have a positive impact on a company. I'd bet that employees who are treated well tend to stick around longer too and that also can have a positive impact a company.

I think research might be a little difficult since there are a lot of things that can impact a company's longevity and success (however that's being defined) that have nothing to do with employees (unionized or otherwise), but I imagine a good study might be able to account for that to some extent.