Fwiw it's not your hardware or video lag created via conventional means no matter what square or nvidia may try to tell you. I have 4 titans and the exact same thing happens to me. I've taken screenshots standing still that have gone up to 60 (Vsync) and down to 30 then back up/down/up/down.
I think it has something to do with the drivers incorrectly changing states between P0 and P(whichever is 2D) compounded with windows 8 gay forced metro Vsync) or whatever it is.
I also get abysmal framerates during the startup screen and while loading screens which aren't present when SLI is off. Points to states changing when they shouldn't be again.
At this point I would strongly suggest speaking to nvidia as I have. I had an issue with no SLI at all that would have gone un addressed until I PMd an nvidia employee. They were able to reproduce it in the QA dept and it went on to be fixed via the most recent release.
During our correspondence he mentioned seeing no one else complaining of issues with ffxiv so I'm not sure if he was blind or limited in scope of research but we have to make ourselves heard.
Between Microsofts bs changes and square insisting on an outdated API, nvidia has their work cut out for them. I can only hope we can collectively get these things addressed.
SLI is no longer a rare benchmark only tech and Surround which they pushed for ages is mature and yet full of problems as well. These are all things that woke well in other games yet they treat ffxiv like it's some obscure unpopular indie game based on what I've seen and heard.
It would be nice to have some help. Complain your asses off. Include data and evidence if you have it so you don't seem like some ignorant preteen who is confusing video/network lag with an honest to goodness issue. Complain to square too! If they get sick of it perhaps they will take it up with nvidia at which point they will be forced to act.
I'll keep you posted when I hear more (my contact had his hands tied w CES)