This is SLI TitanX in the in-game benchmark as tested by Andy of Nvidia:
1620p is 125% more pixels than 1080p so let's assume that you'll have 150% higher framerate at 1080p. At 8x MSAA, that would be roughly 90 fps (36*2.5). Assuming that SLI only has 50% scaling, we would reduce that by 33% to make that 60 fps. Now, we know that the in-game benchmark is way off when it comes to the actual minimum framerate you would reach, namely in the countryside especially at dawn or sunset. If a single TitanX is averaging 60 fps with the in-game benchmark, it will probably be dropping to 30-40 fps in wooded areas in my experience.
My statement was for 1080p60 but I concede that a TitanX could potentially maintain 1080p30. I will note, however, that I was being a bit conservative with SLI efficiency in the above extrapolation. If someone with an actual TitanX could comment on whether they can max out GTA V, that would be great.