It's absolutely disgusting what's going right now with the video cards and it's the reason that I haven't built yet even though I though I would be. You used to be able to get 7950s for less than what a 270x is going for now. Ridiculous. Disgusting. but I will refrain from the rant right now. I just want to say a couple things, first is it is helpful if you list price when you ask a comparative question like this, comparing one card to another, because then the guys with experience can weigh in on it.
The next thing I will say is that the 270X is not a 7870 XT, but a more efficient, faster 7870 that's been improved by improvements with process technology and presumably improvements in the power managing firmware. Perhaps a little bit of AMD's improvements in the firmware of the new kind (on the 260X and 290 290X) have made for some better tweaking of the old stuff. The 7870 and 270X are based on the Pitcairn variant of GCN. The 7870 XT was a trimmed down 7950, which is Tahiti. The 7870 XT was Tahiti LE. Presumably it was done to compete with nVidia at some price war at some pricepoint at that point in time. The 7950 will be better at 2560x1600 and higher and will do AA and higher res textures better. Generally speaking at 1080p the 270X nips at the 7950's heels, or even just surpasses it in some games but not too many, any higher and the 7950 will pull ahead. Metro LL is a game that favours the 7950. I can't tell you whether or not it's worth the $50 at this point. Any price you see right now though (unless you know it's cheaper) is likely to be complete **** except, seemingly, for some versions of the geforce 660.
guru3d took a good look at battlefield 4, a whole article to it. I suggest you go check on it if you have not. sorry I can't give any advice about the 7950s OC levels.