Well here are two reviews supporting this theory of a ~5% difference at the same clocks:
http://www.ixbt.com/video3/rv770-2-part3-d.shtml#p18
http://www.techenclave.com/reviews-a...ew-115201.html
In the first link they've down-clocked the 4870's core to 4850 speed. In the 2nd they've overclocked a 4850.
If there are small 2-3% performance gains from overclocking a 4870's memory, it is due to decreased latency as mentioned above, not increased throughput. On top of this, I doubt they would release the new "cypress" architecture using specified memory modules that create a bandwidth issue that impacts the chips potential performance. Especially since rumored 6 and 7ghz GDDR5 have existed since Feb. 09, they are only using 5-5.5ghz. That would be kinda hair-brained, so the the chip must only be starved for bandwidth when it's being fed at < 150gb/sec. The Oc'd 4850 showing 95% success rate of the rv770's full 800 SP @ 64gb/sec is proof of this theory.
From gurusan (he concludes 7%): http://img127.imageshack.us/img127/7107/36267797qz2.png