"According to sources within TSMC, the 28HP HKMG process is doing very well. So well, in fact, that it supports up to a 45 percent speed improvement over the firm's own 40G process"
^ The sentence is comparing manufacturing processes not GPUs. This reads that 28nm transistors at TSMC can work at 45% faster frequencies as their 40nm predecessors. That doesn't take into account any architectural improvements that AMD will bring.
I would say this time, HD7970 should be faster by a lot more than 45%:
1) Brand new architecture from scratch --> far better DX11 performance, esp. under killer DX11 features such as Tessellation, bokeh DOF. On top of that, scalar architecture makes it far better at geometry computation than the fixed Tessellation engines.
2) Let's say a GTX580 is 15% faster on average than the HD6970. If HD7970 is only 45% faster than HD6970, that would make it only 26% faster than a GTX580 (HD6970 = 100, GTX580 = 115, HD7970 = 145, then 145/115 = 26%).
^ This is highly unlikely because there is 0 chance that Kepler is only 26% faster than a GTX580. AMD can't be naive to think that being just 45% faster for their high-end HD7970 card is sufficient enough to compete with Kepler.
3) 28nm process will allow AMD to add more transistors and increase frequencies. They added 12-15% more performance on top of the 5870 in 6970 by simply doing minor tweaks on the same 40nm process (VLIW-5 --> VLIW-4), added barely any more memory bandwidth and barely added clocks. They were still able to improve Tessellation performance improved significantly. Imagine what can be done on a far more complex architecture that can work on more things simultaneously. Also, I am pretty sure this time they'll add a lot more memory bandwidth.
As rumors point to performance approaching an HD6990, I am guessing HD7970 is 60-80% faster than HD6970 not 45%.