[DRedman45]

My set up isn't exactly in line with what you're looking at, but I do have input on adding ar20's in section 1

First ar20's actually work better than the ar25's with the stock piping, they use the same resonator the ar20's just reduce more. That allows you to cut them to actually match the stock piping

Second, the benefit of the resonator is that it kills the rasp of s54 really well. However it does remove a but of the burble and pops you get when downshifting.

I've been told several times by bmw owners that they can't believe I made the s54 sound so good. Sometimes I would like to try out some rpi's since my coupe is a weekend car and the extra noise would be nice at times, but I love the way it sounds and I wouldn't want to gamble with the money for something I'm already happy with