I know quite a few twin turbo users and they absolutely hated their twin turbo setup and envy our single turbo Legacy.
The low rpm performance with the smaller turbo wasn't exactly spectacular.
During the transition period where the second turbo is about to turn on there is actually a slight hesitation in acceleration as exhaust gas is being diverted to the second turbo while it has yet to spin up.
I have know an owner of a previous generation Legacy who had gone as far as converting their car to a single turbo system from a WRX since they hated the twin turbo system so much.
Subaru ditched their sequential twin turbo system on their newer Legacy for a reason. Still, I guess there might be a slight chance that you can come up with a system that is better than Subaru.