Does anybody what turning off gl extensions does?
Makes it slower (disabling optimization extensions), makes it look crappier (disabling feature extensions)
Do any of this options allow rendering to be in done in CPU only (and not in the GPU)?
No
The engine will NEVER try to do software rendering
setting it to voodoo does nothing
The Voodoo option is only there for the older 3dfx Voodoo addon boards (voodoo1 and 2)