I think the dash, rear seat surrounds, side panels around the PCM, door covers and some other parts are all leather when you have full leather. (The order guide illustrates which parts are leather, I think.)

The P78 option is cheaper than P77 because the car already has full leather option (including seat backs, if you had standard seats). Check the order guide it should specify the exact parts that are leather in full leather.

Full leather is worth it in my opinion.