The only weirdness about such a thing is I presume any rejiggering of Durango specs came in response to Sony's 8GB announcement in late feb.
But nothing about the overheating rumors suggested it's only something that just now came up or is connected to a spec change *shrug*
sony's 8 GB is a reaction to 8 GB of durango, indeed
Anyway, just few month ago Kazuo Hirai tell to the stamp about "we'll not reveal ps4 numbers until microsoft can see it and react", so I think he know that there're room to react
thinking about it, it's very possibile to clock 12 CU to 1 GHz, but 18 CU are not so easy to raise in clock as it would generate an hell of heat
and clocking 12 CU (77x0) to 960-1050 MHz is far far far more trivial and easy than adding 4 GB of GDDR on the main board
maybe they have to tweak the cooling system, this I think need at max one single month of delay, not more, indeed the reveal day shifted to 21, but six months is too much, in six months you can change the whole gpu and relative software library, I think