How do performance parts effect fuel economy?

[Fredrick_NP]

Would something like SMA's Stage 3 performance kit increase or decrease the car's MPG? I mean I realize any effect it did have would be minimal, but still im curious.

And what about SCs? It seems like any kind of forced induction would kill your fuel economy... but I dont know. I know very little about cars, and havent taken interest untill just recently.