V style engines aren't as efficient as inline engines.
Inline engines can rev higher efficiently therefore have a higher Peak torque RPM (higher HP) than V engines
Also the 3.8L is a 2 Valve per cylinder and doesn't flow that well from factory. Add mild cams, restrictive exhaust and the fact that superchargers are not efficient like Turbochargers...you have your answer.
That said a good old 3.8L has more potential for improvement since it is very tame from factory.
GM could have made this engine more powerful. But my best guess would be they knew the transmission was the weak point in the system. It just can't hold abuse at higher HP/Torque than this.
This is why torque management is used on these cars to tame the shifts and torque down.
Also GM knew these were knock prone with the superchargers, which is why it has 2 knock sensors stock and pretty good knock management for it's age.
