The 5800 series definitely has an efficiency issue as performance increase over the 4800 series simply does not match the higher specs. A good example is the performance difference between the 4890 and 5870. I'd expect something near double the performance, but in reality it's nowhere near that.
I've got a feeling that the R600 architecture is slowly maxing out.
You're so wrong now. First of all, these results are here for comparison only, you cannot calculate performance difference judging by a few games. Second of all, you should actually compare Radeon 4870 to 5870 (ATI never decided to make 5890, rather jumping straight onto 6000 series ship). And GeForce 470 to 275.
Here are the results:
http://www.anandtech.com/bench/Product/176?vs=162
http://www.anandtech.com/bench/Product/169?vs=160