Yep, positive. The flat 3" 2-bolt gasket slides past the part where the donut flange would sit on the tail end of the Cobb dp. The part of the Cobb dp where the donut gasket would sit is slightly smaller than 3" so HKS cat back slides right over it without even touching it. That part actually sits inside the HKS cat back slightly. It will work perfectly, I guarantee it. You might want to get two slightly longer nuts and bolts though because I remember the factory spring bolts were a real pain in the @$$ to compress and reuse, but it can be done. And the sound is glorious. It can be reasonably quiet if you're rolling through a neighborhood late at night at low rpms and don't want to stir the cranky neighbors too much, but under normal driving and especially WOT, it's great fun. I really enjoy driving next to Jersey barriers, through town, and through tunnels where the sound can really bounce off of everything. I wouldn't say it's quite as loud as a Harley Davidson, but my family members claim they can hear me coming home from several blocks away. They don't say it's obnoxious, they just hear me coming.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)