Sorry to say that not only have I heard a 996 Porsche with the sport exhaust but I driven one. It sounds like an Uzi filled with farts. I have heard a GT3 and there is absolutely nothing remarkable about the sound. You must be referring to the GT2 which has more of a high performance sport car exhaust.

The very best sound is the CGT. What makes it so special is what you are hearing is the ENGINE! That high scream pitch is marvelous. I have Tubi exhaust on my Spider and though the sound is glorious I would trade it in a nanosecond for the engine sound of the CGT.