Not sure where you're getting your info from but depending on the game either the 5870 or the 6870 can be faster. A 6950 would smoke either one of them. If u don't believe me go use anand's gpu 2011 bench comparison. That's the reason I held off on this deal because the 6870 would be a wiser buy if don't already have a 5870.
I think the 6950 is a all around better card but I wuoldn't say a 6950 will smoke a 5870. I can't find many benchmarks but I though 6950 is faster in tessellation, dx11 games and 5870 is faster at older games.
This is a bit old but showing only 2% performance overall. http://www.techpowerup.com/reviews/HIS/Radeon_HD_6950/29.html