This is a long term review (3 years) after a combined 40,000 miles use on two bikes (Kawasaki 2000 ZX-9R & 2002 ZX-9R Ninjas).
It was my preference to install SuperTrapp exhaust mufflers on both of my Kawasaki sport bikes, replacing the stock mufflers. If that wasn't possible, my next choice would have been Muzzy. My first consideration was performance, second was sound and third was price. The SuperTrapp universal only cost $185, whereas full exhaust systems were out of my price range without mentioning, difficult to find for a bike model almost a decade after production. I bought the SuperTrapp 2 1/2" Inlet Universal Aluminum External Disc Muffler.
The results have been very gratifying for me. Fit was a little tricky, but I was able to fabricate what I needed to make it slip over my 2 3/8" OD exhaust mid-pipe and a local weld shop made the inexpensive hard mount TIG installation a snap. What I got was a bike who's exhaust flow and dB could be adjusted or metered to the performance I was searching for, rather than having to tune the bike's jetting to match the muffler. That was huge in my book. Performance peaked on my 899cc sport bikes at 9 discs of the 12 that were available. The exhaust note is loud enough at slow and fast speeds, without being so overpowering that neighbors complain and LEOs pull out their dB meters.
Power characteristics are better than I expected and I'd say I had high hopes for a free lunch. I ended up adjusting my jetting slightly from stock, but it didn't require the purchase of a jet kit or dyno tune to sort out. Getting the bike tuned just took a little experimentation using the prescribed method of adding or removing 2 discs at a time and determining if power was better or not. The bike has a lot more grunt with the most noticeable improvement above 5,000 RPM. Not expected where several other positive benefits: crisp throttle response, easier starting cold or hot, a powerband that feels like it's always in its sweet spot, virtually no throttle bogging when ridden over 9,000+' ASL summits and enough power from my 900 to keep pace with the newer bigger bikes.
The SuperTrapp on a 4 into 1 makes a pleasing howl at freeway speeds. I think it kind of sounds like a B-29 SuperFortress droning down the highway, but it's when it's revved up that it really howls. Kind of like a baritone blow-torch. That's how I would describe it in race mode. It's a beautiful sound.
Going almost unnoticed is how the adjustable muffler affects fuel consumption. Yes, the muffler is lighter and feels like it's making noticeably more power than the stock muffler, but it's also doing something to help reduce fuel consumption. Based on the numbers I've seen, about 4 - 9% better than how the bike ran stock, however when using 10 or more diffuser discs, economy does drop off. Either way, more flow means more fuel or a leaner less powerful mixture. Using less, say 6 diffusers, will cause fuel economy to be even better, but then top end grunt suffers comparatively. In the bike's current form & tune, it's efficiency will allow it to consistently outrun other bikes, including other 9Rs. Sometimes it'll be by 70 miles to the tank and other times it will be a complete routing of the competition. No matter what, the SuperTrapp always affords my bike a clear distance advantage.
I considered installing their internal disc muffler on the second bike, but ended up forming my purchase based on the safety record I felt I was getting from the external disc system. SuperTrapp mufflers are tunable and are deceptively free flowing. The radial acoustics of the external disc muffler gives the impression that there is a high dB bubble surrounding the bike, without being obnoxiously loud far beyond it. The acoustic dB bubble feels like its about 50' in radius, depending on speed and RPM, which means that highway vehicles within that range will hear a pervasively strong exhaust note at all angles from the bike, even without seeing the bike. It could be conjecture on my part, but there is the appearance that drivers are aware. I don't get the impression cars can't hear my bike's exhaust note. Equally important and what could also be conjecture on my part is the reaction seen of roadside wildlife. In the three years I've been operating my bikes in mountainous canyons and passes, I have yet to see a deer, bear, coyote, dog, etc, make a move towards my oncoming bike. They typically perk their head up, look in my direction and run perpendicular to my heading with their tails down.
Would I recommend a SuperTrapp muffler. Absolutely, I have bragging rights that I've earned! They are definitely considered 'old-school' and unfashionable, I won't try to deceive anyone of that. I hear it periodically from new riders. "Why are you still running that? People are people and they tend to prefer trendy labels. I'm not into labels or trends. I'm into riding all the way up to the point where they start making excuses for why their bike wasn't able to outpace or outrun mine
, then kicking back and enjoying the moment. 
Having a SuperTrapp on my machines lets me do that every ride.
