That's a good question! From what I've been able to gather from readin' things I think .... it depends.
For example, there used to be graphics cards with disabled CUs that you could flash to be the fully enabled version of the card. Whether it worked probably was another matter - if it was disabled to maintain market segmentation or stay within power it might. If it was genuinely a defective chip then oops, better hope you can flash it back. I think you could attempt this as late as some of the RX5xx parts.
As I don't think things like CUs can be individually gated, my guess is that the disabled elements in products like these were just constantly leaking as if they were idle.
In the case of parts with more complex or more targetted power gating, like a CPU, you're probably leaking a lot less with firmware disable parts. But probably still some. Again, there used to be some AMD CPUs that allowed you to attempt a bios reactivation of disabled cores. I think you could try this up to around the AMD Phenom 2. Not seen any talk of this for a long time though, so I think that's long in the past.
Fusing off or lasering off disabled parts seems to be common now, and in theory this should allow complete isolation of elements. Given how power conscious computing has become, hopefully this is what's done now. But I suppose you could just fuse or laser off whatever's needed to reactivate something and let the rest leak. Would seem silly though.
Sorry for the lack of a real answer. My speciality is vague general, answers (with errors).