the 7870 is approximately 25%-30% slower then the 7950. You might get close to a 20% overclock on a great 7870, however the double d isn't a good 7870. so yes. you can overclock a 7870 up to almost a stock 7950. a 7870xt can overclock within a percentage point or two of a stock 7950... however if you're going to buy a 7870xt you might as well just buy a 7950 and get all the overclocking goodness with it.