I'm not sure, tbh. My gut reaction says no, they can't actually do that, but I don't know of any thing that would specifically bar them from doing it.
As far as using studies that have been performed before, my understanding is that what's being proposed is more of a meta-review of existing data, rather than going out and actually collecting new data, so it would be using the existing studies.
As for censoring games and government rating systems and whatnot, those have been fairly conclusively established to be in violation of the first amendment. The only think I could really see happening would be the courts changing their approach to obscenity. Currently obscenity rules only apply to sexual content, not violence, and I could see the court saying that in certain contexts violence can be considered obscene and thus susceptible to regulation by the government. However, even if they say that, in order to qualify as legal obscene content must meet all three of the following criteria:
the average person, using contemporary community standards, would find that the work, taken as a whole, appeals to the prurient interest whether the work depicts or describes, in a patently offensive way, sexual [or violent] conduct specifically defined by the law whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value
It's that third one that's the killer. Almost anything has literary, artistic, political, or scientific value. About the only things that qualify under obscenity rules is actual porn, that is just about sex (with sometimes lipservice paid to a plot). So even if they expanded the definition of obscenity to include violence, I don't think it would be any more detrimental to games than the current version has been to movies or books.
Also, something a lot of people don't understand is that in the US movie ratings are not government enforced, and legally cannot be (except porn under the obscenity test above). There's no law that says a theater can't let unaccompanied minors into R-rated films, or that a store can't sell the same movie to kids, it's just a lot of places voluntarily restrict themselves through company policies.