I have done a ton of research on this subject and thought it would be helpful to a lot of member's here if I shared some general knowledge.
There are 3 general ways to quiet down an exhaust system on a vehicle. Restriction, Reflection, and Absorption. Guess what OEM uses. Unless it's an exotic car it's likely restriction. Reflection is what some aftermarket companies use but on the top end of the RPM range they will actually restrict flow as much or more than stock. Absorption is what exotic car manufacturers and high end muffler manufacturers use. On a medium level production line the reflection style muffler is the cheapest to manufacture.
Let's touch on the subjects of restriction and backpressure. Most engines need some backpressure in order to scavenge properly. Backpressure and scavenging are 2 different things yet they work together. Everybody has a friend who has violently shouted, "you need some backpressure to make power!" But has anybody ever thought about why you need that backpressure? Without backpressure you wouldn't have scavenging. Scavenging is what makes power, not backpressure. That's why you lose power by putting on a huge cat back but leaving your stock manifolds and y pipe.
Take my truck for example, how can I get away with true dual 2.5" tubing with flow through mufflers? The backpressure and scavenging has been greatly improved by adding Long Tube Headers. All the backpressure this engine needs is in the headers since the exhaust is able to scavenge so efficiently. In other words at peak volumetric efficiency the headers actually 'siphon' the exhaust out of the head. Unlike stock manifolds which have much less scavenging, the exhaust needs to be pushed out of the head into a high backpressure low flowing manifold.
The next most restrictive area the exhaust needs to overcome in a stock exhaust system is the 'y' pipe. Whether crimped to nothing like stock, or crafted together professionally by an exhaust shop the 'y' pipe is VERY restrictive. Then hit the cats, the resonator, the muffler, and finally the huge crimp bend to go up and over the rear end. Each of these things by themselves are very restrictive not to mention when you put them together on an entire system.
Exhaust flows exactly like traffic on a freeway! Put 4 on ramps like 20 feet away from each other with no runway and expect the merging cars not to slow down traffic(manifold). Then line the lanes with 6 foot high poles just far apart enough to squeeze your vehicle through.(cats) Now merge 8 lanes into 4. (y pipe) Then make 20 off ramps on each side of the freeway and no runway's.(resonators) Put a maze of different roads going all different directions but ending up in the same location to exit the maze.(muffler) Finally put a huge curve with 2 less lanes.(up and over the axle.)
If properly built the exhaust would flow like 4 long sweeping on ramps, (headers) Each bank would never merge but instead flow out of the tailpipe using long sweeping curves that never lose a lane.
The above analogies are EXACTLY how exhaust flows. For all you hydraulics experts out there, you can also picture the exhaust as water traveling through pipes. The OEM didn't design the manifolds to lose power, they didn't design the crimped 'y' pipe to lose power, nor the resonator, muffler, tailpipe. They designed them to make the system quiet and lose the least amount of power possible while targeting a specific decibel at a target price on a large scale production level! So in other words, cheap, quiet, fast. I was always curious as to how much was lost so I built my own system using S&S Longtubes, 32 horsepower to the wheels! Mileage has also increased as long as I don't drive like a jack-***.
Since most aren't going to spend the money on a full race exhaust system, I'll stay focused on the 2 systems that would make the most power on these trucks. I have tried almost every configuration possible with dyno testing on some of them. I will write part 2 of this when I have time.