Denver is AirCare Colorado emissions testing. You need either your stock downpipe with the two cats or a special high flow cat that 5250 Performance can install for you.
With a traditional high flow race cat, like the kind that come on aftermarket downpipes, you can sometimes get by if you go through the mobile testing stations on on-ramps enough times, but most cars will not pass the rolling test.
That means every two years you have to swap in your stock downpipe, so when the stocker isn't on, you aren't emissions legal.
If you get one of the new technology race cats, you are emissions legal 100% of the time and it is no restriction to power.
2005 Outback FMT