I think it is deliberately not possible to move to 8-bit as a desktop option. XP only allows older apps to force the issue because otherwise their palette switching tricks simply wouldn't work (without some sort of palette switching emulation that would seriously impinge performance).
Performance-wise 8-bit addressing will be far slower on modern architectures, not faster. While you would in theory be pushing less data around the graphics chipsets are designed to operate in 32-bit blocks to addressing smaller values actually takes more effort. Also most application objects will still be processed as a true-colour image only being translated down as needed before being pushed to the screen's frame-buffer for each update.
Another common reason for dropping colour depth used to be limited graphics RAM. As a 1920*1200@32bit image is less than 9Mbytes no modern graphics card in a desktop machine is going to struggle to hold that even accounting for fancy effects and memory consuming techniques like triple buffering and some window having its own large surface processed separately as a frame-buffer by the GPU.