I went with 15 GFlops/Watt as the top-end of what I could imagine a customized and improved Turks GPU delivering, which got me to the 450 GFlops upper limit. The point was to show that the pre-launch "3x" rumours for the GPU aren't really viable even under ideal assumptions, given what we now know about die sizes and power consumption.
Alternatively, if we want to talk about "ideal" assumptions we have to consider the fact that we don't know what process the GPU is made on. The most likely bet is certainly 40nm, but 28nm isn't completely out of the question, and a wide R700-based chip on a 28nm process operating below 500MHz would give much better efficiency than any of AMD's GCN-based GPUs, which are more transistor-intensive and are all clocked in the 800MHz-1GHz range. A 28nm chip is unlikely, yes, but it can't be ruled out without hard evidence to the contrary, and any ideal case would have to take it into account.