SLI is really just a sales pitch. It's like 8 cores in a computer. You don't get extra performance from it, you just spend more money.
While you get insane FRs now, you'll only get maybe 15% more fps with the second card, at more than double the cost (counting price of high-end SLI motherboard and high-end PSU with dual PCIe plugs and extra connectors, etc).
I say live with the empty slot. Use the money on a faster CPU or more RAM or water cooling. You won't gain at all by wasting it on another vid card when the one you have is already getting "insane" FRs.