<You mention the D700/D3 has a a known (but minor) issue for IR conversion. What will the D600 have?>
According to a Lifepixel video, there is an IR light used to make sure the shutter speed is correct on the modern cameras. When you remove the IR filter, at high ISO levels you get some light leakage. This is not a frequent problem as I don't normally use an IR camera in low light - but I have done that some. The D7000 has changed the wavelength of the IR light to above 850 nm - and that apparently resolved the problem. I am not sure what they did on the D800, but I am guessing the higher wavelength will be used on the D600. If not, it is only an issue at ISO 1600 and higher.
Good point on the warranty. That means I'll need to test the camera pretty well before investing in a conversion.