I would expect a 6930 to match or beat the 6870.
your comment about a 6930 being on par with the 6850 is what makes no sense. why on earth would they have a cut down top end card that is no faster than a 6870 much less a 6850?
the 5830 was crap but it was still faster than 5750 or 5770. if the 5830 would have only matched the 5750 while costing more and using more power then that would be the same scenario the op is presenting. a 6930 only matching a 6850 makes no sense whatsoever.yeah, because the 5830 was faster than...uh, I mean, it was very power efficient...er, dang, hopefully they just won't make a 6930.
the 5830 was crap but it was still faster than 5750 or 5770. if the 5830 would have only matched the 5750 while costing more and using more power then that would be the same scenario the op is presenting. a 6930 only matching a 6850 makes no sense whatsoever.