All processors since the Pentium have thermal management in one form or anther (eg. Speedstep, etc). In fact, Turbo Boost in the latest processors is an application of this technology that actually uses thermal headroom to auto overclock. Your processor will most certainly throttle itself if the cooling system fails it, but the cooling system in the SP Mac Pro's has been designed for 130W TDP processors like the W3680 so you won't see throttling unless the cooling system is not performing optimally (eg. fan failure or unusually high ambiant temps etc.).