Sorry guys but I am a little stunned about one of the comments saying that the exhaust makes little difference. They are crucial, otherwise there would be no industry and magick needs to be used. Without back pressure you are going to burn out exhaust valves in no time at all.
Go see a specialist or a performance tuner. Do you want fuel economy, driveability, or all out performance? But be warned, witchcraft will be invoked :-) And it makes a huge difference and can make is sound bloody awesome as a nice little by-line.
Smelly