This past Friday I went to a presentation by Joe Taylor K1JT the principle software designer of FT8, JT65, WSPR etc. It was very interesting stuff.
But he specifically mentioned that FT8 is not designed for a contests like Field Day and should not be used for FD. The ARRL agrees with this viewpoint (I called to ask their opinion).
At the moment they are working on a Beta version called "FT8 DXpedition Mode" but it's not ready for full release. After it is released (they've had three trial tests already) a "FT8 Contest Mode" will be developed. There will be a 2 to 3dB penalty in sensitivity since they'll have to accommodate a longer string of characters (wider bandwidth). A small price to pay during a solar minimum. So maybe next year.