A4 exhuast hot section rebuild

[68PearsonRenegade]

Hi folks,
As the title suggests I'm rebuilding the hot section of my late model A4. I dropped it into my Pearson renegade yesterday. I've perused westerbekes catalog and priced out the flange, I've also perused moyers. I've had wonderful experience ordering many items from Moyer and I thank them for their continued support of the A4 engine.
My question is, what is the advantage to back pressure fitting on the MMI flange? Also is black pipe construction of the hot section the most cost effective. My old hotsection appeared to no have the water entry section either?? Either that or it was corroded and fell off when I removed the system. What is the value of that? I guess I'm trying to wrap my head around the exhuast system and how it functions. My hot section goes to a wet exhuast.