I agree with dannat. Having used the 80-400mm on the Nikon FF, I have found that it was mostly used at the long end. While this doesn't make the lens much smaller, it would be easier to keep the optical performance at the sort of quality we have become used to from Fuji.
I have gravitated to Fuji from the Nikon D800E simply because the FF rig ended up being too heavy to carry over extended distances. However, I am retaining one of the Nikon bodies and the 500mm prime lens simply because there is no alternative in Fuji's lineup.
The aperture used in the proposed long lens from Fuji needs to be bright enough to ensure good AF performance, even at the long end of the lens. If it can't lock onto birds in flight, it is not worth buying. On the other hand, if it can, I'll be the first in line to smash my piggy bank to buy one.
OIS is not important to me. I shoot all my wildlife shots in manual exposure mode at an ISO setting high enough to gove me 1/2000sec or faster to keep the wingtips sharp. Camera shake becomes irrelevant at that speed.