You should buy a down pipe from one of the vendors here.
It will be cheaper then replacing your catted DP.
Then get the car street tuned by a local Tuner. You'll get better mpg and more power.
Most of us do this any way's. You can get a Cobb AP and go to stage 2 map with the DP or get the cable for your lap top and go open source tune.
My tuner is a Vendor on here, www.tuningalliance.com
email him from his site or PM him from his thread in the Vendor Forum here.
You should also check the turbo for shaft play.
What have you done for routine maintenance on the car, are you sure it's the cat ? This is not a normal issue with GT's.
178,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. CHECK your oil, these cars use it.
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