Hey Craig,
I'm not sure whom I heard it from but I kind of remember that they were saying the difference between the 996TT and the one with X50 package comes into play from the 3rd gear up. fromthe get go 0-60 I believe they said the X50 performs they same as the regular 996TT but after 3800 or 4500 RPM it starts to take off.
I'm sure RC will elaborate more on this.