Passmark is a synthetic benchmark that scales perfectly across 8+ cores, and as such plays to the only real strength the FX chips have, high core count for a low price. In practice, most applications simply don't scale nearly that well across that many cores, for games this is especially true, with most titles not scaling beyond 4 cores, and the ones that do often showing little to no improvement when you put a 4 core CPU up against a 6 or 8 core CPU.
Most of the time, the i5 6600k will be better than any AMD FX CPU due to better per core performance and most applications simply not being parallelized enough to benefit from the higher core count. If you're building a rig specifically for video encoding and not much else, then the 8 core FX chips are more attractive, though you could certainly make an argument for getting an Intel Xeon E3 instead, which will outperform the FX CPUs while costing about the same once you factor in the FX's need for a higher end motherboard with more robust VRMs and aftermarket cooling.