Well, the question is kind of moot, because I haven't dared to re-enable UEFI boot since. Supposedly newer BIOS's have the bug fixed, but the value to me in risky another range of motherboard replacements is just not worth it. I suppose if I cared about UEFI it might be a good idea to try it before the maintenance contract runs out, but as a kernel developer, I'm constantly replacing the kernel, so using UEFI is a PITA anyway.
> Is there more security added by enabling UEFI, or keeping it disabled?
You're not really disabling the UEFI firmware at a technical level. You're just telling the UEFI firmware to load a UEFI BIOS compatibility shim, which then proceeds to load a unverified bootloader instead.
How much security do you expect to gain from that? I don't think there's any evidence this will help make your machine more secure against software based attack Of course there are no absolute answers, or otherwise we wouldn't be having this discussion.