In my opinion, AMD's memory controller architecture isn't anywhere near saturated with 2GB of 5Ghz+ GDDR5. Hence why performance improvements are quite easy to come by though increasing just the GDDR5 frequency.
In addition, the IC latency is identical between the 2GB and 1GB engineering samples according to my conversations with AMD. Maybe some board partners will use different ICs but AMD's reference boards all use the same specs.