I never understood the request for "empirical studies" to prove this or that in software development, e.g.:
"Show me an unbiased experiment with measurable outcomes that proves that TDD makes a dev team more productive. Until then, I'll continue doing what I do now."
Let's flip that around:
"Show me an unbiased experiment with measurable outcomes that proves doing [whatever your current process is] makes a dev team more productive than doing TDD."
Unless you can do those experiments (which are extremely difficult to do), you can't use the claim that since there is no "TDD works better" study, that means that it must be worse than what you're currently doing. That's what philosophers call an "appeal to ignorance".
TDD might be worse; you might have a great process! (In which case, I'd love to try it out!) On the other hand, TDD might be better, and you're missing out on improving. Without the (possibly impossible) empirical evidence, you have to try it and see.
