I am planning on getting a Mazda 6 TS2 5-door 2 litre. I know specs are different here in the UK (in fact I don't think it's even possible to get a 2litre version in the US) but listed among the optional accessories is a Twin Sports Exhaust. Does anyone know if this actually makes a difference to performance and/or sound? Or is it purely decorative? It certainly looks good (better than the single exhaust).

Here is a link on the UK Mazda web site for the accessories list:

http://www.mazda.co.uk/sycomax/uploadpool/documents_UK/forecourts/brochures/Accessories_Mazda6.pdf

If the engine is an inline 4 then I highly doubt the dual exhaust has any benefit and is just for show.

4-cyl engines actually do better with a 4-1 exhaust. You get better exhaust gas scavenging (pulling of the exhaust gas out the engine by the previous engine pulse.)