No idle promises
I already had a cone filter fitted so it's made no difference to the sound except that it now sounds like a "proper" five cylinder. MPG? I don't keep a track so I'm not going to make some daft claim; I just hold out my wallet and say "help yourself". As others have written, the big improvement is to spooling and driveability. These things are always subjective, but my feeling when I first tried it was that there was a little more power but that the power now seemed effortless. The hiccup as it came off idle has disappeared and it's very much smoother up through the rev range. When I can afford a remap I hope to get the full benefit as getting all that rubbish out of the way of the exhaust gases can only improve flow. Now I've decatted I'm having a straight through back box; I've got the box, I just need to find time to fit it.