Thats untrue - positive displacement supercharges have zero lag they run a bypass vavle so that there is little/no flow throught the charger. As soon as the bypass vavle is closed you have boost. Also it is possilbe to control boost on most posivite displacement chargers as manufactures generally run a double sided vacuum actuator - so all you need to do is remove or apply pressure to vary the amount of air that is bypass from the compressor. We often do this on the VS-VT V6 superchargers.
The biggest compliment to the FSI engine is that it is direct injection so that higher compression ratio's can be used as chamber quenching is optimised. Another neat idea is the clutched supercharger as this will reduced the forces required to drive the charger once the turbo is brought into action.
To me the best thing nissan could do would be to introduce direct injection, crank the compression ratio up to 10:1 and add varaible cam timing.