i had done the same set up on my car, and where i cant say exactly how much of an improvement it is... i can tell you that before the muffler was removed it was very difficult to heel toe shift it took as many as 3 flicks of the gas pedal before it would rev to appropriate rpm, now it jumps right up on the first kick of the pedal.

it definitely helped free the engine to rev more freely, i am thrilled with response and noise, sounds much deeper in person, nice clean sound

 

very interesting. i think a lot of people here would do their own setup since its cheap and effective