You are better off keeping the stock exhaust and putting in the AFR+ EFI controller by Dobeck. It will show you exactly what your air/fuel ratio is, and automatically correct it based on your baseline settings. When running the stock exhaust, I run +5% fuel at mid and heavy throttle, and +/-0% at idle/low load. I change it to +5% mid and +9% when running the Delkevic. I am way down at 800-1300 elevation. Background to qualify my opinion; exhaust I have tried.... >Stock exhaust -Perfectly fine, no need to remove other than increased noise or reduced engine braking >Stock with catalytic converter removed -No real change other than more unburnt nitrates exiting exhaust >Stock with catalytic converter removed, spark arrestor shortened -No real change, but slightly more bass to the exhaust note >Stock with only spark arrestor shortened -No real change, but slightly more bass to the exhaust note >FMF Q4 with stock header [+/- insert] -Annoying without insert. Not too bad with it. No performance change other than engine braking is reduced >FMF Q4 with powerbomb header -Annoying with and without insert. No performance change other than engine braking is reduced and low rpm torque suffers. Usable power is up higher in rev range. >FMF Powercore with megabomb header [borrowed, did not own] -Annoying with and without insert. No performance change other than engine braking is reduced. Does not change where usable power is in the rev range like a powerbomb. Easy to burn pants/leg on "football" header. >Big gun EVO R full system [+/- EVO insert and custom made inserts] -Annoying, and incredibly loud. Insert makes no difference other than turning it from blaring loud to raspy loud. Low quality construction and weak springs to hold pipes together. >Yoshimura full system [borrowed, did not own] -Annoying and incredibly loud. High quality construction but intolerable noise level. >Delkevic 18" S.S. oval with Delkevic header [header is same as stock, but stainless steel] -My current exhaust, but I remove the Delkevic and use the OEM muffler on the Delkevic header when on multi day trips or at offroad parks. Not as loud as FMF Q4 if insert used. About same as FMF Powercore when insert not used.