When will SIP shutdown the computer to prevent overheating?
System Integrity Protection has nothing to do with controlling what happens if the CPU breaches it thermal limit.


  System Integrity Protection is a security technology in OS X El Capitan and later that's designed to help prevent potentially malicious software from modifying protected files and folders on your Mac. System Integrity protection restricts the root user account and limits the actions that the root user can perform on protected parts of the Mac operating system.




All the CPU's used in the Early 2015 MacBook Pro, Intel Core i5 or Core i7 (5257U, 5287U, 5557U) have a 105℃ TJUNCTION* rating. 


* Junction Temperature is the maximum temperature allowed at the processor die.


While 100℃ is high and getting close to the limit, it hasn't breached the limit yet.