Those benchmarks were done with custom 7870xt/le/myst and a reference 7950. A 7950 vaporx with boost is a different game altogether. The extra 3.5k is charged for the 1gb, 128bit, more stream processors, better cooling and better/higher OCing potential. 7950 vaporx wouldnt sell if there no substantial advantage over the 7870xt.
Tweak the game graphics settings, raise the resolution and check the performance difference. Due to the above extra features, the 7950 will outlast the 7870xt by a decent margin.
7870xt is for those who cant extend their budget to the 7950. Its pointless taking the 7870xt when the 7950 can be had for the specified budget.
Tahiti LE cards by different vendors have identical clocks(Base clock-925MHz Boost clock -975MHz and 1500MHz Memory clock).Even with a hd7950 boost the difference between hd7870XT won't be that big though there would be some difference.Many hd7950 with boost have a base clock of 800MHz and boost clock of 925MHz which is as per the boost BIOS released.Here is a comparison of Tahiti LE with hd7950 at same base and boost clocks in some games:HARDOCP - Additional Apples-to-Apples vs. HD 7950 - PowerColor Radeon HD 7870 MYST Edition Review ,thing to note here is that hd7950 is clocked higher than its normal base and boost clocks for comparison.
Sapphire Vapor-x w/ Boost has 850MHz Base clock and 950MHz boost clock and would be faster than hd7870 XT by a decent margin.
For ~3.5k more you get more VRAM,25% higher memory B/W and more performance.But if you want a better cabinet or some other components then you can go with sapphire hd7870XT.HD7870XT looks very nice for the price.