I have the early 2015 rMBP 13" 3.1GHz i7 with Intel Iris 6100 GPU. It's slightly faster than the Intel Iris 5100 GPU in the 2013-2014 rMBP 13" for OpenGL. Oddly, it's much slower doing OpenCL (LuxMark).
Neither matches late 2013 or mid 2014 rMBP 15" with GeForce 750M for OpenGL Game performance.
The most impressive feature so far is the faster flash storage. Large sequential transfers are blazing fast at 1400MB/s READ and 1500MB/s WRITE. That's even faster than the late 2013 Mac Pro 'tube' flash storage. Apparently the new Samsung SM951 flash has replaced the XP941 flash.