a single turbo will always be more efficient than two, case in point are the fastest supras all are running single turbo setups. problem is finding room for such a turbo, which is why the twin smaller turbos are the route most go.
you could always run a "Sneaky Pete" 50shot of nitrous to aide in spooling such a beast and maintain a always cool air charge.
from what i understand, you can set it up so taht one turbo gives boost at lower rpms, and the other gives boost at high rpms, to get rid of turbo lag.