But targetting more CUs and more ROPs means you need more bandwidth, which means you need GDDR5. Pretty sure they the reason they went with DDR3 was because it was the much safer bet to get to 8 GB, and that meant ESRAM, which limited what they could fit for GPU and CPU. It's not like you pick your memory at the last minute. They would have picked DDR3 and ESRAM as their design a couple years ago. Projecting the price and availability of GDDR5 in that time frame is most likely why they picked DDR3. In hindsight you could say it was the wrong choice, but they had to choose years ago. If their projections were 8 GB of DDR3 + ESRAM or 4 GB of GDDR5, then I can see how they ended up where they are now.
Says a lot about their management if that was their prediction ...