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A couple of threads very recently have discussed this.
http://forums.overclockers.co.uk/showthread.php?t=18465021
On this very page![]()
Most say they will use same size chips so probs 10-15% max difference. But you'll pay a lot more on release so think about is it worth it.
@IC3 I doubt it'd be a new architecture, they'll want to at least keep "GCN" for more than a single generation. Most architectures are around for at least 2 really and across multiple manufacturing processes which bring improvements to that architecture in the form of efficiency and power consumption meaning the core can be doubled up (for example) whilst keeping power consumption, die size and heat output down.