Moved to Exhausts.
As for the question, the technical bods will be along shortly but as I understand it a sports exhaust will be freer flowing than OEM and were it not for the ECU would produce additional horses as a result. Some like the Nismo are designed to look good as well, other's don't have aesthetics as a priority. Lots of R&D goes into designing and testing these things and I would be most concerned at a welder "knocking one up" so to speak. I suspect you could end up with power loss or chavvy sound. At the very least with a decat you won't pass an MOT and will probably trigger the warning light on the dash.
Even one of the cheaper end aftermarket jobs would be preferable IMO.