Yes, less available bandwidth per card. I would have to track it down, but someone showed only a benefit when running very high resolutions, otherwise the FPS loss when running something like a GTX1080 in 8x mode was only a percent or two. Having two cards in that scenario is going to be quicker regardless. Just that SLI and Crossfire are going to be completely up to the developers going forward in DX12, or linked and unlinked (non-matching cards) mode multi-GPU.
You can get 16x/16x with a i7-5930k+ or i7-6850k+, but that would only be needed if you were wanting to run 3440x1440@75-100Hz, or 4K or something like 5760x1080. But even if not it wouldn't be too big a deal.
Odd you should mention my build, ordered a few things and will be putting a few finishing touches in tomorrow. (White PSU cables) Just updated the BIOS and had to re-overclock the CPU, runs a little warmer now, so I am considering de-lidding it. (Previous BIOS was rock solid on voltage with low LLC, now it needs a lot more) And rebuilding my 4770k system and trying my hand at hard tubing, if it turns out well I might redo the new rig.