What palaban was saying is that the HKS is a full downpipe that replaces both rear cats. It is like a combination on your picture there or the downpipe and the midpipe.

Therefore you don't need anything else in combination with the HKS downpipe, you will already be catless asuming you have done an uppipe as well.

No catback exhaust will replace the rear cat. Hence the name catback. The only thing it will change is you muffler and the resonator chamber. Its the farthest rear two pieces in your picture.

I'd replace the front cat or both before doing just the rear. Many people I've spoken with, I spoke with Nathan at TurboXS who dyno tested the differences, and my pal who's done it and disliked the results all agree that the performance gains are to be found freeing up the first cat.