Make sure your checking your speeds with AMD Overdrive. CPU-z only refreshes ~5 sec intervals, may be catching only the 2.8 ghz speeds. AOD monitors the speed at real time.
the only way to work around the dynamic change in the cpu is to overclock it. On the screen where it has C1E, disable it. On the MIT section, motherboard intelligence tweaker, change core performance boost to disable, and change the core clock ratio from auto to between 20 and 22 to start off with. The reason you disable core performance boost is your going past that point anyway. With it enabled, it will spike the voltage on your cpu, you don't want that.
For stability testing change the system voltage control to manual, go down to cpu voltage control and set it to about +0.05V to +0.1V. Then its stability testing to make sure it runs ok.
As for updating your bios, do this before messing with any of the settings. I have flashed many gigabyte boards. I always did it in windows with @bios found here. http://www.gigabyte.com/products/product-page.aspx?pid=3809#utility
Edit: just saw the post from the cpu support list. Interesting that the 8100 only shows 2.8 ghz. you might still be able to manually force it to 4.0 with what I said.