an 8 core AMD CPU when overclocked to around 4.8GHz, can perform similarly to a core i7 2600k (and even beat it in many cases when the task is highly threaded (mainly workstation stuff that can efficiently use 8 cores)
the core i7 still has the better per core performance, but the AMD makes up for it by the 4 extra half cores.
depending on the board, if you crank up the vcore voltage, pll voltage, htt voltage, with a good chip, you can get a decent overclock on the bus, northbridge, htt, and hit a CPU clock speed of above 5GHz on the 8 "cores"
Benchmarks will always show poor scaling because these CPU's don't have 8 true cores, it is basically clusters consisting of 1.5 cores each (kinda like intels hyperthreading but a little more effective)
if you want core i7 performance out of an 8 core AMD CPU, then overclock to around 4.8-5GHz and you will get mostly the same performance (though you will only be getting about the same performance as a core i7 2600k on stock settings)