What are the key performance differences that set the Saucony Endorphin Pro 3 apart from its predecessors in a racing environment, specifically in terms of responsiveness, energy return, and propulsion, and how do these differences impact the overall racing experience for athletes competing at various distances and intensity levels, and are there any notable trade-offs in terms of support, stability, or durability that athletes should be aware of when considering the Endorphin Pro 3 as a racing shoe?