The Powerkit is not the same as the 'S'. The 3.6 is about 325bhp from memory, the 3.8S about 355bhp, if you fit the Powerkit to the 'S' (option X51 for an extra £8k?) it takes the S up to 380bhp, or the same as an old GT3.
A powerkit S is an rare choice, not least because it costs not far off a GT3, but it makes a great 'normal' 997 with comfort/options and rear seats thats probably almost as fast as an old GT3. It would make a great roadcar (especially in C4S form), to complement a GT3 or GT2 as a trackcar. It would depreciate quickly though as I bet the powerkit adds little to residuals........