I wouldn't waste the money on new muffs, sounds good enough with the res off. New muffs won't do anything for performance anyway. Like you I only have the K&N filter. You can get the K&N CAI set up for about 250.00 and Jegs or other places. Alot more bang for the buck than just welding new muffs on.
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2007 GXP Stealth Grey Metallic
Best Beltronics Accelerometer Times so Far: 0-60: 5.40 1/4: 14.22 @ 98.6