All of the reference 5800 and 5900 series cards have an aluminium back plate already
It cools the memory on the back of 5970 and also serves to keep stress off the PCB.
The hole on the back plate is there because the coolers mounting bracket is there, and there are also many components on that area.
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Anyways, an interesting thing about that GTX 480 shot is half of the cooler under and after the GPU looks as hot as the GPU itself.
Efficient heat transfer or heat soaked cooler? Take your pick guys.