The so called dual exhaust can be fitted to any Fiero since the Cat is in the same place regardless. I say so called dual because it's not a true dual system, only one line out of the cat to the muffler with two tips coming out the muffler.
My 84 Indy came with dual from the factory. It makes it louder, not necessarily better. At Idle it's decent, but at speed it's just an annoying drone. As for low restriction, my recommendation is don't bother. The duke likes some restriction and it's not like you are going to gain any horsepower. You are rocking less than 100 hp no matter how you slice it.
As for the Ocelot, I've not heard much good about it. It's mild steel and will rust away quickly. Given the iron duke there will be no performance gain so I would say don't buy the Ocelot.
[This message has been edited by jscott1 (edited 04-28-2015).]