No way that rumored 2560CC will be barely 20% faster than 980 TI, it would be ridiculous.
980 Ti is 2816CC or 10% more than 2560, so for a 2560CC GP104 to be 20% faster would require it getting 32% more performance from IPC increases and/or frequency increases.
If GP104 boosts to 1480 like GP100, that alone would be roughly 25% more than a stock 980 Ti (which boosts to about 1200 on average), so it would then need 5-10% more IPC to end up at 20% fast in total than a stock 980 Ti. So far there has been no indications as far as I can tell that Pascal is bringing any IPC improvements as far as gaming is concerned (all the news have been about compute), so I would say it is prudent not to expect too much here, at this point in time.
Also 20% faster than 980 Ti would be about 17% faster than Titan X, not 10%. There really isn't much of a gap between those two.
Edit:
If we compare to Titan X instead of 980 Ti we get much the same. Titan X boosts to about 1125 on average, so a 1480 GP104 would be ~32% higher, not 40%. So you have 32% higher clock, 17% fewer cores, and 10% higher IPC: 1.32*0.83*1.1 = 1.21 or 21% faster than a Titan X, which would be the same as ~23% faster than 980 Ti.
While I don't disagree, the move to 980 was on the same node, on a mature process, and rather incremental. Something similar here would be almost a total fail, unless they positioned it as a true 'mid-range' product and not a Ti model. The name matters.
I very much doubt the average buyer judges the value of a GPU based upon which node it is manufactured, so I doubt it would have much of an effect on Nvidia's sales.
Sure it would probably be quite poorly received on forums like this one, but we do not make up the bulk of customers.
Also I'm assuming here that the top end GP104 would be named 1080 and not 1080 Ti, since a 1080 Ti performing similarly to an aftermarket 980 Ti would look quite bad.