haha, that's the exact combo I have on my 07 WRX: Cobb catted dp + HKS Hi-Power Carbon-Ti cat back. They work very well together using the flat 3" 2 bolt gasket that should have come with the cat back. Don't use the factory donut style gasket since it's not really made to work with the flat connection on the HKS cat back. I would also recommend getting a new gasket for between the turbo and downpipe. It's not terribly expensive and generally, it's just a good idea to use new gaskets. I'm sure some people will argue that you don't need it, but I got one since it's layered and it crushes when you torque it. I didn't want to have to go in there twice if I discovered an exhaust leak.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)