Firstly , that looks the the dogs mate . How much ? Only thing i can suggest is that the bore of the pipe going into the back boxes is smaller than the original size , 63mm is the original iirc . If it is smaller you would be getting a slight restriction .
When you say it is slower , is it hell of alot slower , or just a little bit .
Or it could even be too big , there is an optimal size , if it was too big then this would have an adverse affect on the back pressure , which fcuks up how the gases flow .
Get under the car and measure it , see what size it is .