I'm 99% sure that all WRX's have a cat in the UP while all STI's do not. Regardless, some STI owners have even put in Crucial UP's since they're ceramic coated. But I don't know how great of an investment that is. I've seen a guy with an STI sware his turbo spooled faster with it, but who knows for sure. But for a WRX, I totally recomend an UP.
I run exactly what you described, "3in dp + 2 1/2 mid + 3in catback"
IMO, if you're going to stick with the stock turbo, the stock 3rd cat won't be much of a hinderence. Basically the difference between 3" all the way and running the 3rd cat is about 5-10 whp.
You could run a cut-out or swap in a racepipe for racedays if you wanted. Hell, you could even get an extra map on your AP for a racepipe and race gas. There's endless options.
For a smog check, PM me and I'll tell you where to go.
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Slow and low, that is the tempo.
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