The power difference between an off road x and a high flow catted x will be absolutely minimal! I don't recall where, but i remember reading a article where they did test this on a mustang, and there was 1-2 hp difference.
Basically a catted mid pipe will be smoother and deeper sounding, with much less rasp and popping. It obviously won't be as loud as the o/r pipe, but will be significantly louder than the stock setup.
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2004 DSG Mustang Gt, BBK long tubes, BBK catted h pipe, SSS Terminator Catback, K&N cai, Steeda UDP's, Accufab 75mm tb/plenum, FRPP aluminum ds, Motive 3.90 gear, Alternative Auto custom tune, Steeda Tri-Ax, DD black bullitts w/315's Nitto 555R's, & 100 shot N20. 269/305 na, 373/528 N20
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