http://www.anandtech.com/show/5818/nvidia-geforce-gtx-670-review-feat-evga/5
Aside from the fact that there are loads of cases where the 670 wasn't nearly as far ahead as you believe, you're all comparing very different situations.
The 580gtx had nearly no architectural improvement on the 480gtx. Nvidia spent a year trying to fix the 480gtx, it always was a mostly basic respin. The 670 brought a very large, and long coming architecture update from Fermi. This time around Maxwell is that architecture boost, it also had the hardware scheduler and compute stripped out... the new parts on 16nm will have both of those added back in and won't be nearly as big as an architectural shift. So the chances of gaining the same amount as effectively a 'full' 480gtx to a 670gtx, and a 980ti to a xx70 is nearly non existent.
That is before we get to the fact that people are basing their assumptions on the 970 price and going with £300.
680gtx, $500/£420 actual price(though on average at launch more like £450), 670gtx $400/£330. 980 $550/cba'd to check £, 970 $329/£275 ish for some apparently.
So lets ignore that a 670gtx wasn't actually £300, let alone £270, it was a good chunk more on a MUCH cheaper process. The 970 was cheaper because it was a much more heavily cut down part that performed way worse in comparison to the fullfat part than the 670 did.
670 was about 10% tmu/shader reduction, same rops, same memory bus as the 680. The 970 had an almost 20% drop in tmu/shaders wile rops were almost 15% lower and don't get me started on the memory bus.
The actual price for the full fat part went UP, the 970 was cheaper because it was much more significantly cut down.
So add this all up, Pascal isn't nearly as big a architecture change as Kepler was to Fermi, not by a mile. It will have to add things back in stripped out vs Maxwell cards, this will negatively impact overall gaming performance increase as it will take up die space. The 670 was £330, not £300 and the 16nm finfet process is significantly more expensive than 28nm planar.
Expect $550-650 for the new xx80 part and $425-500 for the xx70 part, imho.