Well not just those cards are good. The DirectCu for the 6850 in CF run much better (not by much but they do run better) that 460's in SLI. That overclock to about the same.
Low temps don't always mean stability as well. It also has to do with how the person OCs. I can keep my 5850 below 70 running the fan 100% and it will run stable. It performs at the level of the 5870 (a little over) once overclocked. http://www.techpowerup.com/reviews/ASUS/Radeon_HD_6850_Direct_Cu/31.html