I know this is an old thread, i'm a few years behind and trying to make the most of what i can scrolling through all of these...
Is it worth doing this on a Astra, I may already have the pipe now, or should i try to return it and make one to fit?
I am guessing the gains are there though with the other standard mods:
de-precat
de restricted air box + k&n
I also have a cat back exhaust to fit , plan on getting a sports cat and now know (as of about half an hour ago) I should get the Intake pipe into the air box