Aye, it doesn;t do a thing unless its a highly tuned engine. I think 5th Gear did a test on a Clio, Golf GTi (1.8L Turbo) and a tuned Impreza and it was only the Impreza that showed a difference in power. Not even the Golf did, let alone the Clio.
I found the 5th Gear report on power - http://fifthgear.five.tv/jsp/5gmain....143&pageid=272
No mention of MPG tho