I have an 06 6.0 also, put the MBRP 4" turbo back exhaust on and I really didn't see a change in mileage. Although I suspect yours to go down sence you like the sound of that baby. Ha Ha!! I have been very pleased with mine. Hope it works for you for a long time.
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2006 F-350 6.0L 4X4 AFE Intake, Banks Technicooler intercooler system, turbo back MBRP 4" exhaust, 70 Horse Depaco injectors, FICM and ECM/TCM Roll back, Gryphon programmer and Custom Tunes on the way 
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