So I just bought a used 2003 mini S. Sounds like the most cost effective HP mods are 15% or 17% pulley, One-ball exhaust mod, CAI (my car already came with this so, no need to spend money here), and Colder Spark Plugs (why is this recommended, and if so, can I do it myself and which specific plugs do I use?) and maybe an ECU tune.

If I were to get a pulley, and One-Ball exhaust mod done, who in South OC (I live in Mission Viejo area) or North OC (I work in Fullerton area) who do you recommend sell me these products and install them?

Thanks!!!