There are many sources of data on Examiner performance available. Many are subscription based. However, the data is from the Federal Government and should be available to the public already and now sites are showing up to give free access. One example is the Examiner Ninja.
Such data can be helpful during prosecution but care should be taken as every case has its own peculiarities. While patent prosecutors have been using this type of data for years, it will be interesting to see if litigants start to use it, or are able to use it, in court. Consider a plaintiff facing an invalidity challenge who puts forth data that they had an extremely hard examiner at the USPTO (e.g., very low allowance rate compared with others in their art unit). Such information could be quite persuasive, assuming the litigators can get it before the jury under the Federal Rules of Evidence. Likewise, consider a defendant alleging invalidity who puts forth data that the Examiner for the patent is an easy examiner (with a very high allowance rate relative to others examiners in their art unit).
Such data may be prejudicial and have a host of other evidentiary issues, but it is to be seen whether litigators have or will press such issues.