By O2 sensor do you mean O2 eliminator?

FYI making 400whp without going to an alternative fuel (and bigger injectors which are really easy to install or meth) might be kind of difficult.

I'd personally stick with my recommendation above and possibly do the factory exhaust mod. One of the less mentioned advantages of going aftermarket with the exhaust is less weight though... its pretty easy to shed 20+ pounds by swapping out the catback.

 

Oh yes, sorry I meant O2 eliminator.... ok so the uicp mod will work good even on stock ic right? and what is a 3 port solenoid you were talking about? And my goal is not 400whp, I was just using that as an example of what i'm not trying to do... I just want more pep than stock but keep a stock look, that way it'll be a bit of a sleeper.

 

For whatever reason the upper ic pipe change has always been good for 5 -10 whp and is well worth the investment. Yes, it works just fine with stock IC and most aftermarket IC's as well.

The 02 Downpipe as they are called is a downpipe that also replaces the restrictive factory 02 housing (this is the portion that joins the turbo hotside to the exhaust downpipe in a stock configuration) and downpipe with a single part... these are also shown to make a solid 5 - 10 whp over just a downpipe replacement.

The 3 port solenoid replaces the factory dual solenoid bleed type boost controller with a single 3 port direct acting boost control solenoid (this will need to be changed at the time of the tune or else you will over boost like mad on the stock tune due to the nature of the change) which allows for far greater control over the boost curve when properly tuned.

02 downpipe and high flow cat should not make the car much louder at all. If it were me, I would consider an 02 downpipe and a resonated testpipe rather than a high flow cat.