They might somewhat lessen the volume, but I can't imagine it would be by much. The Borla "intercooled" tips I saw on Jegs were an interesting concept on resonated tips. Some people in those reviews swear by them and some say it's a gimmick.
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2011 V6 Race Red - Airaid CAI , BBK 73mm Throttle Body, 3.73 FRPP Gears, BBK Longtube Headers , BBK Off Road X-Pipe , Borla ATAKs , SR Performance Strut Tower Brace, MPT 91R Tune, Powerstop Rotors and pads, JLT Oil Seperator, Koni STR.T Struts & Shocks, Eibach Pro Line Springs, BMR Adjustable Panhard Bar, J&M Caster Camber Plates, Mickey Thompson Street Comps on 19" Laguna Seca Hyper Black Wheels, Several other aesthetic mods