Understanding GDDR5 clamshell mode
In normal mode you can split the lines into 8 group of 32 bit and drive 8 modules x 1Gbit (128 MB) = 1 GB of total frame buffer memory.
  
  In clamshell mode you can split the lines into 16 group of 16 bit and drive 16 modules x 1Gbit (128 MB) = 2 GB of total frame buffer memory.


"The lines" that you're referring to here are lines of the data bus, not the address bus. The address bus is multiplexed in GDDR5 SGRAM.

In normal mode, each memory module has its own 32-bit data bus. In clamshell mode, the memory modules are grouped into pairs; in each pair, both modules share address (and control) signals, but have separate 16-bit data busses.



Since the combined width of all data busses is the same either way (256 bits), the throughput is identical.