Giulio
With a sequential twin turbo setup, you don't always have the exhaust flowing through both turbos, on the legacy a valve closes off the second turbo until over 4500rpm, this means that a small turbo thatcan build boost from under 2000rpm gets all the exhaust flow, and that a larger turbo for high flow and revs can be brought in later.
If you use a single turbo that produces lots of boost at 7000rpm you won't get much boost below 4000rpm
Paul