From what I've read around on I'd say that you could potentially see up to around a 15% fps drop from the true maximum scaling by running it that way. It depends on the game. I haven't actually seen anything GTX670 specific so my thoughts are based on this review of a 7970:
In any case the overall fps you'll gain will easily outway the potential bottleneck in my opinion. Some people will say that 2x 670s are overkill for 1 screen but I have 2x7950s which are similar in performance and I don't feel that I've wasted my money - instead everything is butter smooth.
It is certainly not worth upgrading your cpu and mobo for unless you've got an issue. I'd just wait until you know the platform is holding FPS back (either by having too weak a CPU or by slightly gimping SLI performance). It will probably be a year or two before you feel either of those situations arriving, second gen haswell I'd imagine.
There's not much else I can add other than offering to try out the difference between pcie-3 and 2 on my own pc for you to get some more figures if that helps. It would of course be on a pair of 7950s though.