We should also take into account that the 180% performance of a module compared to 2 normal cores is not a comparison of a BD module vs 2 Thuban cores.

The comparison is quite clear, if they wouldn't have chosen the module design and went for the classic core design, than 2 Bulldozer cores would mean 200% while a Bulldozer module means 180%.

But the saving in die space and power consumption is big so the choice of using modules is obvious.

So they allow a small performance loss, but they gain the ability to add a lot more modules/cores to a BD CPU. Which is really nice.