5850 / 5870 --> 6850 / 6870 was all at the same process node. Performance was really not all that different for these. They re-jiggered architecture some, but really the result was not very different, just silicon size was different so they could have a new silicon size for the 69xx cards.
This is more like 4850 --> 5850, which was a very large jump
I'm expecting 7850 to perform around 6950 level for midrange card price and power consumption. Similarly 7870 vs. 6970 comparisons.
The leaks are not new on this, but seem to indicate that's in the ballpark:
http://www.nordichardware.com/news/...-new-architecture-and-xdr2-rambus-memory.html
7870 is expected to be in the 6970 ballpark with 120watts TDP? yes please! Pricing is the only question. How they've been acting lately has me suspecting they will keep prices up where they are for competitive performance. I'm worried a 7870 is going to launch close to $300, and will only come down once the 79xx cards come out. Time will tell.