This also could be a good and cheap alternative to a wide angle prime for a Nikon D800 for shooting landscapes.
I would love to test one myself. I am a big fan of Samyang lenses, they are bright, crisp and sharp. However I find - and your mileage may vary - that the distortion on their wide angle lenses is worse than some other lenses. To be specific, less than some zooms, especially some lower end "all purpose" zooms, but more than say an older prime Nikkor in the same range.
On a smaller sensor DX frame, usually - and again, your mileage may and will vary - I find the distortion not as bad. But on a big, full frame FX sensor?
What might make this Samyang a great lens is the size of the image circle. Most lenses have an image circle that barely cover the intended frame, and most of your worst optical distortions, as a general rule are on the outer edge of the image circle. One reason so many older FX prime lenses look great on a DX camera. It's also an old trick in large format shooting - the sharpest lenses I use on my 4x5" usually will cover 8x10".
Anyhow, in order to achive the wide range/coverage of tilt & shift, I suspect the image circle on most TS lenses has to be pretty big. So dead on, a good TS lens, even on a FX frame, you would think should be pretty sharp. So if this new lens is sharp, dead on, with the D800, that would not surpize me. the 'acid test" would be how it looks in the corners when you start pushing movements to the extreme.