Alright, here's the update. Got my hands on the GA-880GA-UD3H Rev 2.0 board today. Flashed it to F7 BIOS and still no luck unlocking the cores on the 555BE that's on it, which I will now refer to as Igor.
Swapped Igor with the 555BE I've lovingly named Betsy, which unlocks all cores on an Asus M4A89GTD Pro USB3, and as a x2 OCs to 4.0GHz on stock, a x4 to 3.8GHz on stock, and as a x4 to 4.0GHz with a VCore bump to 1.40V (1.42V with load line calibration).
Igor was very problematic on the M4A89GTD, the auto-unlock struggled. Eventually I managed to unlock 1 extra core, but only after underclocking to 2.7GHz. Not a very good bin I guess. Even then I wasn't quite stable in Windows 7 64 bit but I didn't bother to do a full battery of tests and fiddle with it too much.
Betsy went into the GA-880GA-UD3H and after enabling core unlock, 3 reboots, and a reset, success! 4 Cores stable at stock. Currently in the process of running OCCT overnight at x4 with VCore 1.4V (1.408V with LLC) @ 4.0GHz.
So I'm going to hypothesize here that the Asus boards just do a better job of unlocking low bin CPUs than the Gigabytes (at least on the new 8xx chipsets), and they both seem to do a decent job on higher bin CPUs.