Hi-flow cat converters will volume it up much more, see for yourself with this truck for example. Hes got hi flow cats with a 14" si/do Magnaflow.
http://www.youtube.com/watch?v=EwDOFlPKce4
Are your tips polished stainless steel? or chromed? Chromed tips are junk, nothing beats stainless. You could try sanding and painting the interior of the tips ith flat black paint to help stop the rust.
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2004 Ford F-150 XLT Supercab 5.4L 4x4
Appearance: Gloss Black Grills/valance/wiper cowl, 20% Tint, LED Reverse & Plate Lights
Performance: 2.5" True Duals X piped, Rear Cats Deleted, Magnaflow 11" Bodies, Magnaflow 4x18 Rolled Tips.
Suspension: Bilstein 5100 Leveling Struts & Rear Shocks.
Videos: Start Up, Revs, Driving. Fly-By Sounds Burn Out Video Interior Sound/Drone Tunnel