I worked in that area for quite a while at the GM Proving Grounds in Milford. The horsepower rating is the peak power at the max allowed rpm. When you draw a power or torque curve, the 'feel' you like is the rate of change of the horsepower (first derivative). Its the slope of the power curve that gets you the launch. In most cases in GM vehicles, power must be limited by computer management because of transmission or ring gear durability issues. When those issues are resolved, the fuel is often opened up if Marketting can swallow or buy into the CAFE loss. If they can't, then Engineering may investigate how to get into a lower Weight Class by use of aluminum structures or reduced content.
The EPA also has a hand in the power delivery because emissions is THE major player in power design. The motor's first 20 seconds of warmup during the EPA test cycle defines the performance allowed, because that's when the engine is dirtiest. Fuel economy goals have to be met too. In fact, extra raw fuel must be dumped into the motor to heat the converter ASAP to get the emissions requirements met. That extra fuel is expensed as a fuel economy penalty for the powertrain rating printed on the sticker.
I spent many hours in meetings and EPA test cells at the Emissions Lab watching this all go down.
