Do hiring committees actually give preference to underrepresented groups?
While I am in a corporate position, our process has some similarity, and I also discuss these issues frequently with colleagues in more traditional academia.

What I have observed is that at the level of faculty or other primary investigator hiring, direct comparison of candidates is almost always impossible: it is always apples vs. oranges because every candidate is qualitatively different in what they bring to offer the department.  Thus there is no opportunity for a simple positive discrimination like "given two equally qualified candidates, pick the underprivileged one."

Instead, any strong candidate will have an advocating faction whose interests they would support, and opposition from another faction whose interests would be better supported by a different candidate. These pragmatic interests are likely to dominate over diversity considerations in the final decision in many cases.

Where diversity considerations are more likely to factor into the process is earlier on, when candidates for interview are being selected. Here, demographic information can be used so that a group can notice, "Hey, we're only planning to invite white males... maybe we are being biased in our evaluation and should look deeper into the candidate pool." It probably still isn't enough to make up for the implicit negatives, however.