From the engine back on an 05 WRX without PPP installed - uppipe with cat, (turbo), downpipe with cat, cat pipe with cat, centre section with resonator, back box..
Remember that removing any cat other than the cat pipe will require a remap. Most induction kits really benefit from one too. At worst get it on an RR and spend £45 to make sure the AFR is within the correct paramaters or you;'ll end up with pistons on the motorway.