The difference is the 7850 is cheaper than the 570, uses half the power, runs about 25C cooler under load, and makes almost no noise, while matching/beating its performance. Yes they like to be OC'd, but then again this is OCUK, where the entire point is to take a cheaper card and make it better than a more expensive one, which the 7850 does handily, while having all the other benefits listed above. Looking around the web I've not seen a 7850 that can't do 1050 core on stock volts, it seems that AMD gimped the card deliberately to fit within a TDP, and in actual fact the cores can go much much higher.
As for your comments on the 7870... yes the 7870 is clocked higher... but it also has far less shaders... swings and roundabouts... one is high clocked/low shader the other is low clocked/high shaders, doesn't make any difference to performance in the long run! Both offer up great FPS in games.
Yeah you can get a 7950 for £313, you can also get a 7870 for £255... that's a £58 (18.5%) saving for very similar performance once OC'd. Not really something to be sniffed at.
Personally having used both a 480 a 570 and a 7850 I don't know why anyone would choose the former two now. Yes they are slightly faster, but at the cost of much higher fan noise, much more power draw, and much higher system temps. For me a couple of extra FPS isn't worth all the negatives the other cards bring. But then that's just my opinion, if you dont' care about any of that then knock yourself out on your 480/570 setups, i'll be busy enjoying total silence while I play
