would have to make sure you have forged internals to make the most of it, then huge turbo that comes in later in the rev range, smaller supercharger to get the boost early in the rev range. Supercharger would have to have some sort of clutch to disengage once the turbo spools up then a potentialy complex intake system with some sort of actuator to change intake from the supercharger to the turbo at a given boost pressure or revs. Maybe this part could be adapted from the 1.4 vw engine?
Sounds like it would be expensive R&D and you would prob only get the same power as a well setup supercharged engine, something else in the engine design would become a limiting factor.
Oh and as for cons, no aircon on hot days