In a car with a turbo, compressed air is fed into each cylinder and burned, because its compressed air that is at a higher pressure than normal air, it means more air is in one cubic meter than usaul

more air in a cylinder, means more fuel is needed for combustion, therefore creating more exhaust gases, thats why a 3" zorst setup is recomended for cars with turbo so the exhaust gases can flow out more easily and less energy is lost trying to force the huge amounts of gas through a smaller setup such as a 2.25"

since u got an 1.8L non turbo engine, 2.25" should be plenty, IMO, 2.5" is overkill,(unless u got heaps of mods other than turbo) correct me of im rong but stok integraz dont come with a 2.25" setup, the stok is smaller, thats y u can pik up some extra power with the larger piping, but 2.5" IMO is unnecesarry and ull prob only get more noise and not more power