It's all in the restriction.
If you have a restrictive exhaust, they're said to now flow at 100% the capability.
A 2" system on a 2L car say, may be able to flow 10L/second of air.
But another 2" system, on another 2L car (Both the same type of car, everything identical, except for the exhaust) may only be able to flow 9.5L/second, this is where you get a restriction, and you end up with a back pressure in the exhaust (If you actually do some VERY indepth reading on how to design an exhaust, back pressure you want equal to zero, or even a vacuum (Which won't happen!)
Therefore, the second exhaust system, although 2", is said not to have 100% flow through it. It is restrictive.
When the guys I deal with, have been building exhausts for 30+ years, they know every little trick, they even research new items for various things, and new designs. I'll stick with them.
They know how each exhaust system should be setup on what engine, and depending on the variables within the engine, they can slightly alter how the system should be setup.
The design is done through theory, and then tested in practice through gas flow benches (And hey, we're dealing with gasses here, not fluid!), through the dyno, and also on the track when the systems are initially designed. And then the varying factors they know how to alter and how far.
Remember, you can't alter systems all that much, as pipes come in set sizes.
As for different materials, a stainless steel system gives alot nice resonance not over that of mild steel, but then again, if you ever get to hear a copper exhaust system (Illegal), they sound the BEST!!!