The MAN-JFK/EWR market is one which has seen substantial growth ex MAN in 2015. The figures (per Scotty Dog of SSC) are as follows:
JFK: an increase of 119,033.
-2014, 112,996 were carried on the route.
-2015, 232,029 were carried on the route.
EWR: a decrease of 2,878
-2014, 114,317 were carried on the route.
-2015, 111,439 were carried on the route.
I think I am right in saying that United are the only operators of the EWR route for both 2014 and 15. I am not entirely sure who the new entrants to the JFK route were, but it may have been DL and TCX.
In broad terms, the route has shown the ability to prosper and grow rapidly. The issue as has been alluded to are the flight times which are all bunched in the morning. I doubt BA will use their own metal to serve this but I would not put it beyond AA to introduce an evening departure (although I assume all flights are morning departure/arrivals for good reason).