I found a website site about exhaust, it talk about how important of back pressure. It doesn't mean free flow is good.
http://autospeed.com/cms/A_108185/article.html


Currently my mod:
L15A stock engine
4 stock injector + 1x 300CC injector
Piggyback tunned AFR 12.1
max boost 0.6 bar, currently start boost 1900 RPM.


History: I had remove my CAT and found that my response is lost.
my low end power totally like tortoise. But improved alot at High power.
But this is not for daily drive, so i installed back the CAT.


Recently: I had installed Bypass Value exhaust after Resonator / before muffler. the bypass v/v cut in at 4000 RPM and i found that my Low end remain good as stock, my mid and high power improve, but not as good as CAT-LESS.


Future: Should I change to high flow Cat? But if back-pressure drop alot, mean less torque = low end power lost. Does anyone installed high flow CAT and didn't found lost of low end power?????????

 

theres a really good read about backpressure right now in this forum, i would definitly check it out.

have you seen the Weapon-R race header? the dyno looks pretty good for catless and itd make even better numbers for boost. its the only race header ive seen for the fit. there have been quality issues in the past, anyone vouch for their recent work?

Maybe you could check out a lightened pulley set, dynos say they provide all their benefits down low. could help offset the lose in low end without the cat?