I've said this in other thread, I will say the same thing in this.
Based on numbers from HD5300 and HD5500 we can assume, that Broadwell Iris Pro 6200 should be in performance on par with GTX670M which is pretty nice GPU even by todays standards. Increasing core amount by 20% and new architecture increased the performance by 60-80% depending on the job what the GPU has to do.
Skylake, which will end up in new MBPs this year will be AT LEAST 50% faster than Broadwell Iris Pro.
Yes, were talking about integrated GPU from Intel CPUs. And the performance of those Integrated GPUs will increase with every next generation.
Why would you need a dGPU if you have that kind of performance from Integrated?