You quoted the 1070 in your post which definitely isn't 30-40% faster than a 980ti... And it would be accurate to say that in some use cases a 1080gtx might be circa 30-40% faster then a 980ti but it's definetly not true to claim this as an indicative figure for generalised performance uplift! (
reviews at release suggested a more like 15 - 25‰ uplift)
I have had a 1070, 1080,1080ti and a slew of Maxwell and Kepler cards running relatively high resolution displays for a number of years.
And if the xx80 cards have all shown 'noticeable improvements' over xx80ti cards what are people whinning about? What with calling the xx70\80 cards 'mid range' and other nonsense