I flashed 'grub_t500_8mb_libgfxinit_corebootfb_usquerty.rom' on my quad-core T500 with a qx9300. Speedstep works fine on my custom Coreboot build (4.13). Libreboot 20210522 is stuck at 1.6ghz (which is the lowest speed GM45 quad cores can run, as they don't have a super-low-frequency mode for 800mhz). Behavior is the same if I boot directly from libreboot-grub, or load Seabios.
I'm willing to spend time and help debug this issue, but I have a very full schedule for the next several weeks.
I believe you mentioned on libreboot IRC that a coreboot ROM gave you the possibility to use those cores at the maximum speed. Do you still have that ROM? If so, please send it to me. I would like to look inside it, to see what coreboot revision you were on.
One of two possibilities likely: configuration option that I overlooked, or regression in recent coreboot 4.14 release. With the above info requested, I can look into this.
Here's a copy of the coreboot ROM I built. I reflashed it on my T500 and confirmed it allows running all 4 cores at maximum speed (2.53ghz in my case). The build date is 7/21/2020. I don't have the original build directory or config files. It boots straight to SeaBIOS, no grub, no proprietary VBIOS, no microcode. The SeaBIOS 'nvramcui' and 'coreinfo' boot options show the first screen but are buggy and aren't interactive. Booting from SATA or USB works fine.
I hope this helps, and thanks for taking a look!
EDIT - notabug only seems to support uploading jpegs and pngs, not binaries or compressed files. I'll send via email.