Looking for ways to improve frame rate.

[Xoriam]

You need to change Transparent lighting from high to normal. <- this one is most likely the cause of that stuck at 50fps problem.
Unfortunatly Nvidia has problems with the way they do transparent lighting, I was in contact with someone from Nvidia for a while who confirmed the bug and said they would eventually fix it, however it's been about 6 months and the issue still exists.

Other options that you can lower without actually noticing a difference are turning off HDR, and turning off Ambient occlusion. (untill DX11 release obviously. turning off HDR will not be possible and AO make a much bigger impact on the visuals with the DX11 client)
Also enable Occlusion culling.

Turn SLI back on, it works fine with FFXIV. it's just the options i've mentioned. Especially the transparent lighting one which causes issues.