If you live in an area that does advanced smog testing.. don't waste your money on a catted downpipe.
I bought a catted downpipe a few years back, and had to go this month for my emissions testing.
At Advanced Smog Testing Facilities they test HC (Hydrocarbons) CO2 (Carbon Dioxide) and NOx (Nitrogen Oxide)
My Catted Downpipe allowed me to pass HC and CO2. But I failed NOx miserably!!
Here's the important part:
I spoke with the state emissions inspector (very nice guy btw) and he advised me that on WRXs... we have 2 cats in the downpipe because we don't have an EGR system. The two cats are used to break down the NOx. One cat would be sufficient to break down the HC and CO2 (just as mine was). But only our stock exhaust is able to break down the NOx.
I asked him how to reduce NOx levels without putting a stock exhaust back on. He said that NOx is produced by high EGTs (exhaust gas temperatures). Our turbo charged cars run a higher EGT than the standard car (1400degrees is common).
The only ways to decrease NOx are with alcohol/water injection, an alcohol fuel source (E85) or increasing the fuel / retarding the timing.
Retarding timing / increasing fuel would allow less fuel to be burned increasing the HC output. Alcohol / Water injection would lower the NOx level, but not enough. E85 would also lower the NOx level... but not down to levels acceptable for a 4cyl car.
Solution? Stock downpipe for emissions testing. And Catless for stage 2+