Would you say 3,6 % is that stringent though? I remember GE under Welch fired 10 % of the lowest performers every year, that seemed kind of stringent. But Tesla is fast-hiring and acquiring, so seem to have a larger need to fix hiring-errors than GE. At the end of 2015 they had only 14,000 employees! In light of that, 3,6 % seems completely reasonable to me. Low, even.
Concerning the performance review: would expect that after acquisition, SolarCity would adjust to Tesla's performance standards. So even if they were based on prior performance reviews, still seems like a reasonable action and level to me.
As much as I try to squint, I'm having troubles seeing the smoking gun here.