Requirements analysis is an integral part of the development process. Giving teams fully specified requirements at the beginning of the experiment wouldn't be realistic.
Depends. You could then run experiments on team performance in the requirements-to-code phase, separate from the generating-requirements phase. That has its place. And then you could experiment with teams trying to convert informal requirements to formal requirements. That might let you learn some things about the parts that you couldn't learn if you dealt with the whole.