I know Nvidia did it for their mobile gfx updates but not sure on desktop gpus. They marketed the same chip with a slightly faster frequency every year on mobile for several years. If you didn't have any knowledge of this you'd end up buying a crappy laptop for much more, when you could have had the same laptop with a lower numbered gpu that had the same performance. This was due to OEM's requiring that gpu manufacturer's come out with new chips every year or some crap like that. So all the did was rebrand the previous year's chip with a new number basically.
But I bet the 580ti outperforms my 680 even! If EQ is all you play, 580ti should be a monster as long as your resolution is not above 1080p (not sure how much vram 580ti has). When you start gaming at higher resolutions you need more VRAM, even then I'm not sure EQ is that much of a performance drainer on the gpu, probably more so on the cpu then gpu since its such a legacy game.
As far as naming x50 = low end performance/price, HTPC type card.
x60 = low to middle range performance/price
x70 = top of the middle performance card
x80 = high end performance
TI = 2 of the same gpus on one pcb (so you don't have to go SLI or buy 2 of the same cards).
After x80 is just enthusiast bullshit like Titan. Waste of money imo.