Did you reset the ECU?
When was your last:
Tuneup? IE: Plugs, Wires, Cap/Rotor...
Oil change?
I'm assuming your header is the USDM piece, HEADER BTW, You have 1 HEADER Not HEADER'S which is the plural form and No B series motor has 2.
Header choice effects power, which is yours 4-2-1 or 4-1, USDM or JDM?
Are you running your stock cat? If so you will feel better power with an aftermarket High flow cat.
The Dunk is a tuner exhaust mostly for sound. A better choice in the apexi lineup would have been the N-1 which free's up the mid and Top end.
Who did the install? Is everything secure? Are all the flanges unobstructed? How does the car idle? Feel in general as compared to before?
The difference in feel from a restrictive system to a free flowing I/H&E should defiantely be noticed Especially with a GSR. Not a bone jarring experience but, certainly a more linear power delivery And Certainly stronger mid range and top end response.
How many miles are on the car?
Have you dynoed the car ever or yet? Dynoing the car before the mods is always the best way to figure out where you stand.
A.