If I had seen this thread earlier, I would have recommended you buy the LX manifolds. They're more efficient than the stock WK's, but you'd require slight head pipe mods. Probably help the fuel economy and the performance a little. Here's a thread I did on the differences:
06 GCO, Dk Khaki, QDII, Tint Ft & WS Visor, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Cbn Fltr, Debadged, Fumoto Drain Valve, BT Catch Can, Chop EGR Tube, SRT-8 CAI, K & N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Rep. wheels.