The Honda CBR 600 F4i muffler definitely flows well considering its design, and is reasonably quiet. It's a multi chamber muffler much like an ordinary automotive muffler, just smaller. I'll describe it as best I remember...Unfortunately I didn't have the forethought to take pictures during that part of the development. I believe it had three chambers, we'll call them 1, 2, and 3 in the order from the front of the muffler to the back of the muffler. The exhaust enters into into chamber 1 hits a heavily perforated plate that bisects the chamber. The open area of the perfortaed plate significantly exceeded that of the exhaust tubing, thus minimizing losses but creating a more even/consistent air velocity. It then flowed into a tube that carried it to the chamber 3, then the next tube carried it back to chamber 2, and finally from chamber two it hopps into the last tube that goes through chamber three and out the rear of the muffler. All tubes inside the muffler had bellmouth ends, and were extended from the dividing wall a distance into the chamber for tuning purposes. The main can was of double wall construction that had a dense layer of insulation between the inner wall and outer wall. I don't recall for certain but I believe the inner wall was also perforated.
For cost, weight, and packaging reasons we decided to replace it with a muffler of our own construction. After a not insignificant amount of development (at least 17 iterations) it finally achieved the same dB level, but was likely down a little on power compared to the stock Honda based on pressure drop tests. It was based on two extended inlet/outlet expansion chambers placed in series. It had a straight through flow path, significantly smaller in diameter than the stock Honda, but required the use of smaller tubing. It also still required a perforated plate/cone much like the stock honda muffler used. In the end it was able to get a 600cc sportbike motor at 11,000rpm to under 110dB measured about 20
inches from the outlet. After extending the exhaust (which was already pointed vertically) to get the exit above parts of the car it was reflecting on, I think our final number was 108dB.
The problem with mufflers is that the primary ways to really get them quiet tend to go against ways to get more airflow and tighter packaging. Basically a smaller inlet is better. A larger body is better. The larger the inlet, the larger the body must be to achieve the same sound damping. Not that there aren't other tuning means that allow one to get more or less sound damping for a given airflow need, but astraight through 4" OD cylindrical muffler with a 2.5" inlet/outlet will not be as quiet as a similarly designed straight through 6" OD cylindrical muffler with a 2.5" inlet/outlet. Thus why external side pipes that look good (not much larger than the inlet/outlet tubes) are going to generally be significantly louder than the really dorky looking solutions, without necessarily performing any better.
The development of a muffler that is functional, cheap, and attractive for the average locost builder to make in his garage would take a considerable amount of time and effort, and would likely end up not working any better than many of the relatively affordable commercially available products already used. You'd basically be reinventing the wheel to save a few bucks. The other option would be to find an existing muffler that works reasonably well and uses simple construction, and "reverse engineer" it (aka steal the design) to be independently reproduced. If the
spiral muffler baffle design were to work well enough, it would probably be one of the better candidates for a locost muffler.
On the other hand, I've often wondered how that same basic exhaust I built would sound if were not so limited in packaging constraints (make it bigger) and run on a normal car engine (not on a high compression, agressively tuned, engine at 11,000 rpm).
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The track doesn't care how much noise you make. They have to measure because the neighbors complain sometimes.
I am surprised at how small the outlets on these exhausts are, so just a little back pressure may be helping.