Mine had been mapped-pre silencer removal and didn't cause any noticeable flat-spot issues. To be honest it's the perfect mod, it sounds meaner on cold-start-up, other than that a fraction louder at idol and a bit louder full throttle but pops and bangs much more...especially with the sport mode button. I just went to Longlife exhausts in Carshalton but any exhaust place like them or RSR etc would do it circa £50 cash for a nice neat job.
It won't change your life like some people on the forums claim but definitely worth doing. I'd do it again if I would have got another one