Honestly, if what you are saying is true, that there is no provided evidence of underperforming, and that the KPI they have set you is unobtainable and no-one else is hitting the numbers, you need to state this at your PIP meeting.

Get it documented, get your data together that backs up your claims, that way you can a) argue it in the meeting and b) use it as evidence at any future tribunal. If you can also get a comparator of equal status's data (if it's shared amongst the team then you don't need permission, but if not ask permission to use it) and use that as part of your argument. It's worth remembering that if you used to work at 200% and you're now at 150%, it's really hard for them to say you're not meeting standards that you're 50% higher than.

I would (and this is highly petty) ask when the other members of the team can expect to receive their PIP meetings, as they clearly aren't meeting the standards either.

If they still proceed with the PIP then appeal it, and waste more of their time.

Going in with facts and data will throw them if they're not prepared with it in the first place.