It says the turret weight is 456#. 456<800.
I don't know why the idea was dropped. I'd suspect money and time, though. An integrated system like that is going to require a lot more development. It's a major airframe change, for one, and for another, the systems integration piece is what always bites one in the ass. A gun like that has to be tied in to a very complicated set of electronics.
Because a system like that would require the nose to be reworked, it would've had to have been integrated earlier in the development process. Since they were already behind timeline, they probably wouldn't want to wait to perfect a turreted gun.
I'd also speculate that such a gun would have an aerodynamic penalty, too.