Those darn ATI chips are "hot", alright: Apple purposely slows MacBook graphics chip.
GPUs are just so huge. It's an "embarassingly parallel" application, so ATI & NVIDIA can just keep throwing more gates that the problem, following Moore's Law. The Holy Grail is to render cinematic quality in real time. We'll get there some day, but there will need to be lots of technology and methodology innovations to use power wisely.