I'm sure the SEAT lads used these on SEATcupra.net and said they did have some effect. Not sure it was where the designers intended you to use them, but they used them as a breather somewhere and it did improve the engine response apparently. The only reason I didnt fit one was because you had to chop up a breather pipe. Someone like Ian might remember more than me
From what I remember a few people reported better MPG & a smoother gear change with it. Personally i wouldn't touch it, I'd spend the money on Jelly Babies first