PT-Tuning has a quote on their site from an owner who got 20 HP increase! from their duals...
" I just picked up 20 HP on my GT. I bought my 2003 PT GT August 1st 2002. The day San Diego county was receiving the first GTs. I was delighted to find a set of Dual exhaust from your web site and just received them 5/1/03. I had them installed at a local performance shop. He put it on a Dyno -- Before installation, 210HP. After installation, 230HP with 260 ft/Lbs Torque. And it sounds "MEAN" ......."
Thanks: Larry Kern Oceanside, Ca."
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http://pt-tuning.com/GT-Turbo/GT-Turbo_Main.html
PLEASE Call Larry and tell him he is a liar if you don't believe him.