Despite Nvidia's declaration that FFXIV does not utilize SLI, I can guarantee that both of my cards are utilized when the game is running. I use MSI Afterburner's OSD for the Logitech G15 keyboard. In fullscreen mode both GPUs and each core of my CPU have utilization stats streamed to my G15's LCD display and all resources show activity. The cards hover around 70-90% and the CPU is around 80% for core 0 and 40-50% for cores 1-3.

Unfortunately, SLI does not help with stuttering in largely populated areas. It's a bottleneck caused by the CPU. The only relief would be a hardware API like Mantle (or DX12) being leveraged by FFXIV's Crystal Tools graphics engine to push draw calls and HUGE batches straight to the GPU instead of through the CPU.