Do you have any solid reason to believe that a hypothetical G81 is going to close the bw deficit (as opposed to marginally narrow)? I don't. So I think such a scenario, should it come to pass, could have variable results across different settings. We'll have to see first with R600 exactly how far up the resolution/settings food chain you have to go for the bw advantage to start to shine.
Edit: Of course, having said that, looking at that ROP/memory implementation they did, I don't see any particular theoretical challenge for them to step up to 512-bit if they are willing to spend the silicon. It's obviously modular, as GTS attests. But are they willing? And at what process will they refresh: 90, 80, or 65 --and the implications for timing.