Something's not right here...The HD 8800M has 640 USP's and 992 GFLOPS of power. The HD 7870M has 640 USP's and 1024 GFLOPS of power. The HD 8800M is clocked slower than the HD 7870M by 100mhz. Same USP count, not a significant difference in power even if clocked up to 7870M clocks...
AnanTech shares my feelings that it is just a rebrand as well
Interesting AMD provided chart as well
About what is to be expected from GCN gen 2 for the 8500/8600/8700 and most likely the bulk of the desktop cards as well, 25-45% improvement range.
AMD FX-8350 w/ Corsair H105, ASUS Sabertooth 990FX Gen3/R2, 8GiB G.SKILL DDR3-2133, XFX HD 7970, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, Corsair C70, HP ZR2440w, Win 7 Ultimate x64