I have been lots of conflicting threads about what seems to add power and what doesn't.

I am considering either just a catback or adding a HFC/O2 DP.

So what is the actually difference in power once you are tuned between the two? Its the $300 - $700 question I have at the moment.

I don't want to read anything about "get a test pipe". I stopped using them because I got tired of having a headache everytime I start the car.

 

the real gain is comes form the TBE . not from the CBE.
So dependent what you are looking for.

 

That was very true on the VIIIs and IXs, but the gains seem much smaller on the X with stock turbo.

 

Not sure how it compares to the Evo 8/9, but for the Evo X, having the full TBE over a CBE is a night and day difference. The gains in both peak power and area under the curve is substantial! The O2 Housing really opens up tuning potential.

You can see the difference here:
http://www.youtube.com/watch?v=ojhnhO1LZo4