As gibby mentioned the guarentee is for the life of the car, as long as annual checks are completed. Oh and you cannot tamper with it yourself.
Regarding the noise tuning, i left mine as free flow, it was fully fitted custom made for the car, 3 boxes. The way they change the sound is to put little restricter plates into the pipe. Circular plates with a smaller diameter hole in the middle and weld them in to stop the gases leaving so quickly.
Again its true what has been said about its all down to the level of competance of the welder. Some jobs i have seen have looked shyte 6 months down the line, make sure you turn up at least half an hour beforethe guy says it will be ready so you can look at it when it is up on the ramps, if you don't like the workmanship then ask them to do it again, your paying after all. The guy i went too was a top bloke and had clean welds all round. Wouldn't fault him.