I compare the latest with the previous fastest.
So you take the only excptions to the rule, 770 was a refresh of the 680, merely oc.
And then you have the boldness to pick the 970, which is on the same node as the 780.
GTX 670 > GTX 580
GTX 470 > GTX 285
GTX 260 > 8800 ultra
8800GT 256MB > 7900GTX
7800GT > 6800 ultra
6800GT > 5950 ultra
5700 ultra > 4600 Ti
See the pattern? ALWAYS the x70 is faster than previous gen fastest.
That is not exactly an apples-to-apples comparison. You will note that in the past, the second tier SKU has been carved out of the same silicon as the top dog, and for the most part you are dealing with a comparable sized GPU from generation to generation. For example, the G80 (of the 8800GTX/U) compares to the GT200 of the GTX 260, the GT200B of the GTX 285 compares to the GF100 of the GTX 470. This trend is unlikely to continue as architectures bifurcate between gaming-centric and professional usage.
As an aside, your timeline is out of whack:
The succeeding second tier card following the Ti 4600 was the FX 5800 (in January 2003). The FX 5700U didn't arrive until October.
The succeeding second tier card following the FX 5950U was the GF 6800 (non-GT) in May 2004. The 6800GT didn't arrive at the second tier pricing segment until November when the 6800 (non-GT) moved down to the $299 bracket
The succeeding second tier card following the 7900GTX512 was the 8800GTS 640M (G80). The 8800GT 256M didn't arrive until very late in 2007.
Regardless of the hierarchy, your examples work because in the past the succeeding chip has been more complex than than the one it replaced:
FX 5800 (NV30, 125m transistors) > Ti 4600 ( NV25 A3, 63m transistors)
GF 6800 (NV41, 222m transistors) > FX 5950U (NV38, 135m transistors)
GF 7800GT (G70/NV47, 302m transistors) > GF 6800U (NV45, 222m transistors)
8800GTS640 (G80, 681m transistors) > 7900GTX512 (G71, 278m transistors)
GTX 260 (GT200, 1400m transistors) > 8800U (G80, 681m transistors)
GTX 470 (GF100, 3100m transistors) > GTX 285 (GT200B, 1400m transistors)
GTX 670 (GK104, 3540m transistors) > GTX 580 (GF110, 3000m transistors)
We are now at a point where this is no longer true. GP104 carries less transistors than GM200. So, thanks to increased wafer costs, likely worse yield prediction, and a huge disparity in die area between GM200 and GP104, it is very probably that you can throw out past examples because the rules no longer apply - especially when factoring in salvage parts...and with foundry costs escalating, and GPUs evolving a degree of specialization depending upon market and workload, we probably wont be returning to "the good old days".