I haven't modded my car at all, but i'm sure that unless you do the whole breathing system of induction and exhaust you will not gain any significant HP. The resistance from your factory exhaust and CAT will nullify the gains from the improved induction. What you will get is a happier engine, slightly more efficient, and no need to replace the air filter 'cos you can clean them!
That said, if you have the money, buy the Pipercross Viper with cold air feed (£250)*, pop a hole in your wheel arch liner with a means to scoop some cold air into the engine bay, fit one of the funny e-RAM induction fans (£200), then replace the factory exhaust mainfold, CAT, pipe and back box with a powerflow job (£600) and hey presto - you have been relieved of £800 - £1000, plus extra premium on your insurance with a gain of between 10-20 BHP! (confirm anyone?).
Marko
* = Prices guestimated at time of post.
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give me ambiguity or give me something else