If you appreciate one performance part alone will not make a lot of difference to the overall performance of your car, but by addressing the intake and exhaust side, you should see a healthy increase in performance most of the time.
I'm going with the Takeda on the intake side, and probably a set of PPE headers along with a custom stainless exhaust. The whole lot should add 40/50HP, but more importantly to me will improve things across the full rev range, and improve the sound. It's not like we need huge HP gains when we have over 400HP as standard! ;)
It's torque and getting a smoother curve that I am looking for. Making the power the car has more accessible.
You normally find a mixed response on forums about any intake system. Loads of them suggest it's more responsive, faster, etc. Loads of them are honest and say they can't feel much difference over OEM, but it sounds a lot better.
Unless you are talking about a turbo-charged car like the Supra for example, you won't get much extra power from add-on's that cost a couple of hundred quid! ;)