Veriable runner intakes are partially spec'd for the reason above, but it also has to do with cold start issues.

If you've ever had a car with a variable runner intake fail, the car will misfire like crazy.

The drivetrain of these cars is suited perfectly for right about 500hp. After than, it all starts going very quickly. They knew the potential of these combination and I strongly feel that the "weak" drivetrain is more of a fuse. It prevents people from bolting on Forced induction or something similar and making a ton of power without a serious commitment to the rest of the car.