I agree, it seems as if Speed Step cannot be disabled by just setting it to Disabled. It also depends on the CPU's C-State settings.
In the Advanced menu, CPU Configuration, find the C-State settings near the top of the options. Set all of these to Disabled:
Enhanced Halt State (C1E)
CPU C3 State Support
CPU C6 State Support
Package C State Support
Of course have SpeedStep disabled too. That should work for you, I use this board with the 2.80 BIOS, same CPU, and I tested these settings.