My question is similar to this. However I need to clarify some details.
I can't find anything about this statement "I also read that the vc4 driver does not use the gpu_mem parameter, confusing things a bit." in the original question.
Moreover, in Gentoo wiki https://wiki.gentoo.org/wiki/Raspberry_Pi_VC4 I see this: "It's also recommended to bump your gpu_mem to 128 (it's what raspi-config does when you enable the driver through it).", that means. that gpu_mem is used on every type of graphic mode.
So we have xf86-video-fbturbo-git and KMS driver. First one is good for 2D acceleration, second one for 3d. But, as I know, KMS driver implements in xorg.conf via modesetting driver, which does not support 2D acceleration (https://linux.die.net/man/4/modesetting), so only 3D will work good but no 2D scrolling. Am I right?