My headers aren't as long or well done as his, but I didn't lose much if any low end and gained a moderate amount of top end. Also I have 2.5 inch piping and a magnaflow muffler and resonator. The closer you put the resonator to the header the more of an effect it will have on the loudness.

ATX MTX swap, rear drum disc swap, drilled rotors, ported intake, optim throttle body, CAI, 7.5L MAF, adj. fuel regulator, longtube headers, underdrive pulley, no EGR, low inpedence bigger injectors, 2002 Focus Swap