My mrs had a cat back powerflow fitted to her car about 5 years ago, They made up the pipework then we went & listened to it & chose the volume & sound we wanted. They tried different cominations of silencers untill we were happy. It sounded good, was well put together & had a lifetime warranty. About 2 years later she hit it off a speedbump ( car was lowered ), took it back through & they sorted it for free as it had just pulled the join apart.
This was the Darlington branch. I must admit I have heard bad stories about them too though