..hrm, mid range power used to pass people? Hrm.. Add those things you mentioned, and downshift.
Hehe.. no really, the simple bolt-ons will increase overall power. Like the breathing systems. The air-intake will suck in more air, yes, but, if you don't straighten out, open, and lighten up the header, the air you pull in will have no where to go. Also.. open up your cat, downpipe, and muffler. Dispite the common "lawn mower" sound, there is a *reason*, why it's open and sounds that way.
Anyway.. go on with the bolt ons. You'll see the power increase. I wouldn't worry too much about internal engine mods unless you find out that the "passing power" you desire taps a hidden urge to push your car to the limit.