taking the lid of the airbox not only decreases HP, it makes the stock paper filter vulnerable to anything being pulled up into the engine bay... that's just f'ing stupid.... HKS does not give any real HP gain in comparison to others. Many of my friends have this, and they love the sound, but lost out a little in the HP range for the amount of money they spent. It also doesn't radically reduce the weight, which IMHO, is the greatest performance increase by switching the exhaust. Bottom line, get what you want. Check out the PRM intake as well... many have found it to be great... or comptech... depending on where you live (hydrolock issues) it could become a life or death desicion...
I have the Spoon Carbon/Kev. duct, a K&N filter, and the Spoon Throttlebody, and my throttle response has improved immensely, but overall HP is not really "dynoable," being that is really picks up when the car is moving at about 60 miles per hour (sorry, I don't know the exact wind velocity)... Be smart about this, and think about what you really want to accomplish with the car, instead of wasting money on parts you are just going to replace in 6 months... ( I am guilty of this...
) You wanna stay NA, supercharge, or go turbo? this should help you out in your desicion making...