Welcome to the forum!
No one has bothered to reply to your query because the subject of exhausts is a well worn track in these parts
There are plenty of previous threads on this subject, some fairly recent, so if you go back thru previous pages in the garage section you will find enough reading to take you into the wee small hours and beyond.
Mild steel, stainless extractors, high flow cats, mandrel bent, single or dual, the list goes on but in the end it all depends on how much you are prepared to spend.
XR8 Chic speaks from experience (she has spent the $$$$) and if you are going to throw big money at your zorst you should follow her lead and consider installing a programmable chip (like a Unichip which is programmed in real time on the dyno) on your car to optimise the benefits of both Horse Power and economy.
When I upgrade my zorst I will be doing before and after dyno checks to able to measure the benefit
Good luck and happy reading!
The cop said "Do you know why I pulled you over". I said " Well, that all depends on how long you have been following me!"