The key is most places aren't as strict as you think they are. It really depends on where you live. Personally, we don't have emissions testing in WV, but they will fail you if you have a CEL on. The CEL will only come on if you have no cats in your system. WRX's have 3. Even if you have no cats, you can always get a MIL fix/02 simulator.
Most places have a sniffer test, but since they don't have AWD dyno's, they'll only do it at idle. Usually if you have atleast one cat, you'll be fine. As for no cats, someone else will have to answer that since I don't have to worry about it. Some people, like in Cali, just put their stock DP back on for emissions testing since they only need to be tested for emissions every 3 years (I think?)
Also, I wouldn't recommend you gutting your uppipe. It's dangerous enough to affect your health that could be avoided with like.. $200 and that still doesn't gurantee that you get all of the junk out of the pipe.
... If they turn you down and send you to a Subaru dealer to get you emissions tested, those people are morons, and I'd try to avoid them in the first place. AWD cars are exempt from the dyno sniff and only require a idle sniff depending on where you live.
I wouldn't worry about it so much. Relax, just read up a little. There have been several posts about these debates on this forum alone.
"We live for a good time, not a long time."
RIP Colin McRae