It was underrated at that level. Technically it could reach C6 ZR1 and Mk2 Viper SRT10 levels of power, but it wouldn't have passed emissions in Europe until the release of the RS version, so Porsche understated it to be "officially" in the league of C6 Z06 and Mk1 Viper SRT10, even though we all knew it was competing with the Ferrari 430 Scuderia and Lamborghini Gallardo Superleggera!
With that being said, Porsche finally lifted the "official" power restrictions on production cars starting with this particular car. That's why I will always love the 997 generation, especially of the Turbo, GT3 and GT2 varieties.
Porsche did the same thing with the GT3 at that time, capable of C6 Z06 and Mk1 Viper SRT10 levels of power (even in independent road tests without modifications) but underrated "officially" at C6 Grand Sport and Maserati GranTurismo levels of power.
To be fair however, the 911 is more of a Nissan GT-R and Aston Martin Vantage/DB competitor. The Corvette, Viper and GranTurismo all aren't exotic or high-quality enough to match the 911 even when they're so "good" performance-wise.
