It all depends on your test suite, of course.
TPU measured ~25%, Guru3D measured 31%. That's still a ways off 35-40% faster than a 580, plus as I said it cost 30% more than a 580. If you were to get 980Ti + 25% and that big a price drop, it would be a great value and they'd sell a ton of them.
Keep in mind, along with the last new node came a pretty big shrink in die sizes. GF110 was 520mm² and GP104 was 294mm², and the transition also had the benefit of moving from a DP compute heavy die to a DP light die. Here we're moving from a very large 601mm² die that's already extremely light on on DP compute, to an unknown size for GP104. An equivalent size ratio would be 340mm².
I'd love to see a GTX at 980Ti + 40% (or Titan X + 35% if you want to use full dies) performance for $500-$550, but I'd be pretty shocked if it happened.