25% more? Have you even looked at how much the 5870s from real manufacturers cost? It’s $50 more, $75 at most, which is 11-17%, for 15-30% greater performance. Even at your lowball estimate of 10-15% higher performance it’s comparably priced.
Oh and there are games the 5970 and SLI/CF configs of the 480/5870 can’t get 60fps on. So yeah, when it comes to the newer games that 15-30% makes a difference. Especially if you’re buying with more than 1 year of intended usage.
Also worth noting, the big gain for the GTX 480 was in the minimum FPS for all the tests, which at times was twice that of the 5870.