According to autoblog.com, the CF tub weight 146 kg (324 pounds), without any of the subframes, and I believe that's quite a lot. Now I don't know how much a 599 aluminum full frame weights, but this is twice the McLaren's weight.

Concerning the AWD weight, considering that Ferrari guy believes the FF could loose about 100 kg without the AWD and the DCT, I don't know if it could explain all the gap. Also, I remember tuner Edo Competition removed the AWD hardware from a Murciealgo, and saved 70 kg, if that's something worth being noticed.