Wow, I didn't know pickers and stowers got PIPs. Obviously you did a smart thing in that you went and got a medical diagnosis. The company would be facing a medical disability lawsuit if they followed through with the PIP/firing.
A lawyer I spoke with suggested employees regularly visit their doctor about work related stress so that when they inevitably get PIP'ed they can claim medical leave and work related illness. Some places its a war zone and that's what workers have to do.