Highly doubt you have a case, because I highly doubt that your numbers are the ONLY reason listed for your PIP. There are always specific behaviors tied to a PIP, and there’s no way to prove that your behaviors are the exact same as someone else’s. Your best bet is to correct every negative behavior listed on your PIP and perform any other action listed as part of the plan to improve, no matter how stupid, and document it as much as you can. It will make it more difficult for P&O to approve you getting fired. That being said, once a manager puts you on a PIP, it’s clear that he/she wants you to leave, so staying will be miserable unless your manager leaves.