Corsa-B.uk - Vauxhall, Opel and Holden Corsa B Owners Club

I've read about people selling powercaps for the X16XE engine was just wondering what exactly they do? If they come as standard on the sport engines? If not how would I go about fitting one?

allows better airflow to the intake manifold, supposed to gain more power
usualy find em on the c20xe especialy if its in a corsa, it would replace the SFI box as the SFI box is too large to fit in the engine bay of a corsa (i belive)

Does the X16XE have a SFI box? What does it do better than the SFI box?