They probably just don't bother to improve 5 series performance because they are EOL. 6870 should never get close to 5870 unless there's heavy tessellation involved. As a 5870CF owner I'm a little pissed that they gave up on improving 5 series performance. Since 5870 and 6870 are architecturally identical aside from improved tessellator it shouldn't be a big hassle to incorporate those improvement to 5 series. They just don't give a damn about owners of their older cards.
In fact, it looks from the Tweaktown article that not only does 11.4 not help the 5 series, it actually hurts it in some cases. Far Cry 2 is the most extreme example:
http://www.tweaktown.com/articles/4046/amd_catalyst_11_4_windows_7_driver_analysis/index7.html. The 5870 went from being much faster than the 6870 to being slower (at lower resolutions), because the 5870 got worse while the 6870 improved. Strange, since this game doesn't involve tesselation, which would be where the 6870 could pull ahead.
What I'd really like to see is a crossfire comparison, as 11.4 was supposed to improve scaling, but there has been an ongoing debate whether these improvements would benefit all cards, or if the 6 series had something unique in the hardware that would allow greater improvement.