More efficiency means that when it comes time to stuff a chip full of transistors till 300W comes out they can stuff more transistors in there. Power and performance are two sides of the same coin.
very well said. perf/watt is the determining factor for how much perf you can stuff into a high end flagship GPU which typically draws 250-300w. perf/sq mm determines the GPU die size required to provide that perf. The goal of any GPU architecture should be to maximize perf/watt and perf/sqmm.