Umm... got it for almost a month, and I really like it. The sound is very dark, but not loud (should have no problem with the cops). The pitch will become even lower if under load. You will be able to set off alarms of other car just by drive slowly beside those with very sensitive shock sensors (sorry to the p5 that parks across me
). As some stated already, the pitch will not change with the rpm, it sounds the same throughout the rpm, only louder. Since I have an SRI, it will overpower the sound of the tanabe once over 4000rpm. In highway driving, you'll have a nice humm behind you, but isn't loud enough to actually make you crazy.
As for performance, my butt dyno tell me that it has very minimal gain in hp. I would say that it has made the engine more responsive, and together with my SRI, high end power is improved, although at a trade of less torque in the low end (more obvious because i have an auto). I'll post some pic together with some additional comment in the Tanabe board once the holiday is over.