The same principles hold true in the exhaust as in the intake "T". If the flow is forced to separate from the walls of a tube due to an abrupt change in direction, it will "separate" from the walls and effectively reduce the diameter of the tube. Smooth, gradual transitions are what's needed to keep pressure drop to a minimum.
I'll refer you back to post #16 in the "T" thread:
http://986forum.com/forums/general-discussions/31693-maf-throttle-body-diameter.html
One aspect that many people don't realize is that the air at the inside of a bend is moving faster than the air on the outside, this is the opposite of the wheels on a car, so it's not intuitive to most folks. Due to momentum, the faster moving flow is more likely to want to keep moving in a straight line so the inside of the bend in a tube is critical to keeping separation from occurring. It's significantly more important to make the inner diameter of a bend large and smooth than the outer diameter.
The exhaust pipes in the pictures will definitely help to reduce backpressure through the exhaust mostly because they make the flow turns more gradual. To what degree depends on the pressure drop in the rest of the system. Considering this is post-muffler, where the air is pretty well cooled off and very turbulent anyway, there's probably not much to be gained.
In addition, they will make a significant difference in the pulsing of the flow between the left and right sides. I suspect the stock system uses the left and right sides to cancel some noise, the designs shown won't be doing as much of that, so they will make the exhaust louder, even without changing the muffler.
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