Extra CPUs will not make 3D acceleration noticeable faster, since it runs in the host graphics card, not the host CPU.
OTOH, if a constipated guest is CPU bound for other reasons (e.g. foreground and lots of background tasks running on one core), then perhaps increasing core count woukd help loosen things up.
This will absolutely NOT fix a "software rendering mode" message. Only installing the GAs will do that, and even with lots of CPU cores available there's no way that software rendering can compete with hardware accelerated graphics.