I'd recommend waiting until you get your AP to install the downpipe so you can flash to a Stage 2 map right away. Otherwise, you might throw a CEL and your car won't be running to its full potential. You can install the cat back any time though if it fits. My thoughts on cheap-o exhausts are that the welds sometimes tend to rust and develop leaks, you might experience fitment issues, and you might eventually have to pay more money to buy another exhaust after you get tired of dealing with those issues. Sometimes people do have good luck with them and they can last a while... it's really a bit of a gamble. Also, if you haven't already done so, be sure to buy a new turbo-to-downpipe gasket and use a fresh downpipe-to-cat back gasket when you install the whole thing. Good luck!
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)