Looking at this page gives the impression that an 8 core steamroller even with added L3 cache would be about the same size as a Kaveri die.
http://www.anandtech.com/show/7507/amd-kaveri-apu-launch-details-desktop-january-14th
AMD said GCN takes 47% of die space. 8 core BD would be about the same size as kaveri.