Okay...
Iris Pro benchmark numbers:
http://www.notebookcheck.net/Intel-Iris-Pro-Graphics-5200.90965.0.html
GeForce GT 650M benchmark numbers:
http://www.notebookcheck.net/NVIDIA-GeForce-GT-650M.71887.0.html
Scroll down and look at Skyrim's numbers:
Iris Pro - High settings : 21.4fps | Ultra settings: 12.1fps
650M - High settings: 41fps | Ultra settings: 24fps
Differences:
High settings:
91.6%
Ultra settings:
98.3%
I was being moderate at 80%.
If you work out your math with the rest of the benchmarks, the difference is consistently 30-50% in favor of the 650M.
And that's STOCK 650M. The one in the rMBP is overclocked to match 660M performance, so it should be another 10-15% faster on average.
I suspect most people really underestimate just how much graphical power the rMBP has. It does have a lot. And to top it off, you can even overclock the dGPU further since the cooling system is much more competent than on Apple's past MacBooks.