The HD6870 is less than 10% slower than an HD5870 with launch drivers and yet has 480 less stream processors,lower memory bandwidth and the core is only clocked 50MHZ higher. The HD6870 has improved tessellation performance over an HD5870 too:
http://www.legitreviews.com/article/1445/14/
http://www.hardwarecanucks.com/foru...286-amd-radeon-hd-6870-hd-6850-review-22.html
hardware canucks shows an almighty improvement of 1FPS..woot.