The calculations dont work as simply as that due to numerous other factors that all affect the flow rate of air in and out of the engine plus inertia of components and compression ratios.
Ill try and break the statements and answer individually for clarity.
The NA vs turbo debate for equal BHP (power) relates to how the power is delivered. The power for the NA engine is there straight away so giving drive earlier. An equivalent power turbo engine has a delay while the turbo comes up to speed (spooling up) and thus pressurising the air intake - this increases the air fuel mixture (due to density) crammed into the chamber giving a bigger bang, this is how smaller capacity engines with a turbo can generate the same power as larger capacity NA engines.
The engine characteristics between the legacy and the impreza are different not only because of the size but also because of the turbo setup and the tune, they are designed to be different drives, nice fat torque spread against a more active driver engagement to stir the gearbox and keep the motor buzzing - the Spec R legacy though could be some what more of a beast so dont count that in this train of thought.
I would advise driving both as I believe (having driven a couple of vectras) you will find both cars really enjoyable to drive but I think you will struggle to draw theoretical comparisons when you start to take into consideration power to weight ratios between the vauxhall and the subarus - its not just about the individual numbers but the sum of all parts