MS doesn't have a good track record when keeping faithful to their technology exhibitions. I never buy into their marketing BS until we get the games out the door and using it.
And secondly, again, I'm not seeing "500%", especially in a catered benchmark tool. I already have a game with lower level access that doesn't expand too much beyond DX11 in terms of performance. EVE Online also has an extremely high ship count (3k+), yet it doesn't bog down like the demo does because it isn't abusing the drawcall functions as much. In fact, the only card to see even close to 500% is the 290X, which suspiciously runs as slow as a 260X in DX11 (and is even more suspicious when a fukking 750Ti runs faster than an R9 card).
Lastly, I look into holes like that and see the marketing fluff is in overdrive. I don't deny there will be gains, because there will.