Mr.Bape

the majority are comestic. some have functional purposes though. like eric has agent 47 window louvers which provides air intake to cool his big brake system. i've also seen louvers that provide the same purposes but for turbo setups in stangs in the rear. though ive yet to see that actual setup.

crashed1st2

1/4 window adds about 5-7 hp. rear louver adds 10hp. big wing-up to 22hp. stripes- 12hp, undercarriage neons-8hp, those stupid windshield wiper spray nozzle thingys that light up neon blue on the hood add 5hp. license plate frame that says honda crx adds 3hp. self painted interior to match car color adds 4hp. racing decals add 2hp.

Thurman

Mainly cosmetic 99%
However, it does help keeping the sun rays out.