from my own experience, reading, and seeing others.
Its the back box that is most restrictive. Was the case on my pd150 anyway.
dp and cat removal will possibly gain 1 or 2 hp, but will
help spool and make the car rev easier, thus making the car a little faster, and will give the illusion of much more power.
I went from a stock system to milltek dp decat non res and twin miltek backbox.
Then later I had the backbox removed. I did notice a gain. However I did have a hybrid, not that it should make a huge difference.
Sound wise, on tick over there was a deeper note. Not crazy, but you can tell the car was tuned so to speak. On normal driving, was same as most uprated exhausts, not boomy etc. However on full chat, it was loud. Plus I had the turbo spool down sound. Pigeon as some call it. I once was asked if I had a external wastegate LOL.