pierrerock :
The is not only clock speed to tell a given GPU performance. A R9 270X will run at 1100 Mhx while a R9 280 will run at 850 Mhz. So you can't compare it to this.
I didn't only refer to clock speeds. I added the core configuration, because the number of shaders etc. is crucial to performance. That's exactly why an R9 280 is more powerful than an R9 270X.
I also referred to the memory, which is difficult to evaluate in this case because of the unusual combination of a small eSRAM cache and DDR3 memory in the Xbox One. The DDR3 memory by itself would be a major handicap for the GPU, but the eSRAM will alleviate that to some extent.
pierrerock :
A XBONE runs newest games from 1080P to 720P at 30-45 FPS with something which looks like medium setting. What can achieve that ? a 750 TI or a R7 260X.
That's a bit optimistic. Besides, there are performance differences between PC and consoles that have nothing to do with the power of the actual GPU. Console games can be optimized for just that one particular hardware configuration, whereas PC games have to be able to run on a huge variety of hardware.