x79 is only beneficial when you have more than one GPU. Two GPUs on one card doesn't count because they are already sharing one PCIE x16 slot. That will be the bottleneck no matter how many extra lanes x79 has. Having extra PCIE lanes on an x79 only helps if you are using up the lanes in an actual SLI or Crossfire configuration where two different PCIE x16 slots are used. He definitely should have gone x79 because he is planning on using two cards in two different x16 sltos. But if he stuck with just one 690, the only benefit the x79 platform would have given is if he paired it with a SB-E processor that was more powerful than the 2700K.
I was under the impression that it would have been beneficial, even with one 690, thank you for the correction!