Fair doo's.
But if you take it to a custom place you will see greater BHP (granted not much) you can have your own tail pipe and it will cost less.
Full custom system normally comes in at around £310 - £350 fitted.
£280 for a irmsher back box unfitted just seems a bit to much to be honest. Have you had a look on ebay you may be able to get a good second hand one for £100.
I have had a few exhausts done here http://www.exhaustsuk.co.uk/ never had any issues with em lights coming on etc.