Considering you're running HFCs and X Pipes, your performance won't be reduced noticeably. Maybe a couple of horses here and there but not anything that you will notice by driving in the streets. The most important part of the exhaust system is the the design in the X Pipes, and the cats. Although its true that for maximum horsepower you do need a very good flowing axle back, its not as important.
:.-. KW .-. RPi .-. SUPERSPRINT .-. MACHT SCHNELL .-. VOSSEN .-. VREDESTEIN .-.: