Re: exhaust/power question i need the 2 cents!
If you're gonna go with new cats, I'd look into hi flow aftermarket cats. Pypes, Magnaflow, SLP, they all offer hi flow cats for our mustangs. This way you're only stuck with 2 cats and will still be emissions legal.
The 2.5" piping is standard on pretty much all aftermarket systems. You really won't lose that much backpressure...or torque, maybe 1-2 ft.lbs. A set of under drive pulleys would recoup that slight loss in torque. They are cheaper than the 99-04 udp's, so I would consider them for your car.
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.