I honestly do not know the answer to your question, how many Performance Shifters it would take. An interesting question.
But I am wondering about your wording here, I am probably not reading it right, but are you also gonna slot the other various enhancements that help with endurance recovery?
It just sounds like you only using PS +end
The global/unique enhancements Miracle, Numina, Pancea and Theft of Essence all help your endurance.