ohhh i found you owned an 04 wrx! see, i was right! you can go to subaru and get an ECU reflash to fix the problem you're having in the 4000 rpm range.
That way you wont have that lagging feeling when you are at full throttle as you were having before. That lagging feeling should be MORE obvious with the turbo back exhaust. Just get on the gas in 3rd gear and when the car hits around 4100-4200 rpms, hold it there... you'll see what i'm talking about :wink: it'll make you want to get the reflash right away.!