GTX 580 used to be faster than a 7870:
https://tpucdn.com/reviews/AMD/HD_7850_HD_7870/images/perfrel_1920.gif
Today?
370 is an overclocked 7850, 270X is an overclocked 7870:
https://www.computerbase.de/thema/grafikkarte/rangliste/
Time is not kind to the old, especially if they have less VRAM.
For a ~1 year place holder sure, but what would you be waiting for since 1070 is here? Cheaper 1070/1080 after more AMD competition? The time for Nv 900 and AMD 300 series placeholders has ended, imo.
Using different reviews with a complete set of different games on different systems- not a good comparison at all.
The next issue- the 270x was 130-140% the speed of a 7850 and 110-115% the 7870 at launch. Which means, it would have always been faster. Busting that bubble.
Since there are so many different 370 models, partner custom designs on a card that was quietly rolled out....it is a little hard to nail down exactly where it stands but logically it should be a a decent improvement over the 7850, especially considering we already had a bump up from the 7800 to 270 series.
But you know...all of this is besides the point when you are comparing an ancient nvidia card vs an AMD line thru 3 generations of improvements. The new consoles have sent ram requirements well past the days a 580 was acceptable...
I mean, seriously, your comparison is not at all a valid when looking at a 480 vs the 970.