First thing I did to mine was replace the centre section. Mines a MY03 wrx with PPP. Was going to do the cat pipe your mate has, but on closer inspection it turned out I had one, just had the heat shields still on it so it looked like it still had the cat!! Unresonated centre section makes the car a little louder, more burbly than all out volume and a few pops on over run. Sounds much better (IMHO) than the standard PPP set up but don't expect to shatter windows or deafen your neighbours, it's quite subtle, but it's there[]
Previously had a classic and did the same, again with PPP, replaced centre section with decat part, keeping prodrive bbox and sounded[Y]. Then changed bbox for another, and much much louder. Point being (again IMHO) what influences the overall volume the most is the back box. Takin out the cats, resonated bits etc will affect the noise/sound of the exhaust but if you consider the amount of sound deadening stuff in the rear box compared to the rest of the system it kind of makes sense. Keeping the prodrive box on the back should keep it quite civilised