From what I've seen of Mantle, Intel and AMD CPUs both benefit from driver overhead and load distribution improvements, with chips like the i3 coming having some of the largest percent improvements. You might consider, rather than a 4690K, getting a cheap i7 Xeon instead, or even going with a 4790K.
The Xeon 1230V3 is clocked 200mhz lower than a (stock) 4690K but has hyperthreading for only a $10 premium, and you can opt for a cheaper H97 board and probably save around $50 between motherboard and cooler, making it a cheaper option overall.
http://www.newegg.com/Product/Produc...82E16819116906
The 4790K is $90 more than the 4690K, but is clocked ~500mhz higher and has hyperthreading, and can probably also be paired with a much cheaper H97 board due to it being near the max clock one can generally get with Haswell without unreasonable voltage. You might still want an aftermarket cooler, but it's again possible to save ~$30 with a cheaper board, making it only a $60 (~20% on the CPU alone, much lower if you consider overall system cost) premium for eight threads and 15% higher clocks.
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Despite how little more it is to get eight threads over an i5's four, though, it's arguably a waste of money (today) because very few games benefit at all from having more than four hardware threads - even console ports, which were natively 6+ threaded on the consoles. Although an i7 can be as much as 70% faster in scientific computing than an i5, in games you'll see very little benefit beyond the clockspeed advantage. It's interesting what that implies about FX chips.
EDIT: The argument for a Xeon or i7 is much stronger if you do any significant amount of non-gaming, computationally intensive work on your computer. I'm quite content to sit on my i5 for a long time though.