The two pieces that will give you increased HP are the pulley and the tune. All of the tuners are pretty good and have decent packages.
As posted above, some are dead set against a tune for some odd reason (yet to be debated as to why) but as found in a previous thread, it does make a difference in the cars performance. My advice with a 15% reduction pulley is a tune to lean the car back out a little. The car will run without it, but not as well.
As far as fitting is concerned, the pulley is the hardest part. If you are planning on doing it yourself, you need to make sure that you have all the tools available. It can be done in about 3 hours if you are handy with a wrench.
Performance wise, you will love it.
