For me it depends largely on the final design of this ride. If RMC is going to change the ride dramatically, than it will probably open 2018. If it mostly follows a similar course, it might hust be a 2017 addition. As another poster mentioned, it also depends on how much length they add/subtract from the ride. If the ride significantly shortened, than 2017 is more likely. If the ride is roughly the same length or longer, 2018 is a more accurate target.
NTAG took two years, but that was RMC's first project and they recently grew dramatically in size. Also, the lack of other projects may mean they are going to try and open this thing by 2017.
There is the avenue that says this is RMC's biggest name client yet, their biggest conversion yet, and the shortest timeline yet. If the discussion was "we will aim for 2017 but no promises" then Cedar Point's excellent marketing team may want to wait to announce it for 2017 untill they know its going to happen ala TTD.