This is a quiet insert that works very well. I did notice a slight loss of top end power, but that may honestly be a perception due to the fact that it is quieter and therefore seems like less power. A dyno is the only way to really tell, another plus is I seemed to gain a little on the bottom end. I noticed a significant decrease in noise, but it still has a good sound. It's not whisper quiet, I don't want it that way. But it's no longer ear-splitting either which is a really good thing. I definitely recommend if you care about helping to keep our riding areas open.
So the item works as advertised once installed. The only issue I had was that the retaining screw they provde with the insert is not the same pitch as the one that comes with the exhaust and original spark spark arrestor . The original screw that comes with the exhaust cant be used with the Quiet Core insert as its not long enough. I had to go to the hardware store and find a correct pitch/length screw. Then when I went to install, I and several others attempted this I might add, we could not get the screw to go into the hole in the exhaust while the inert was installed. It threads in when the insert is out so I know its the correct pitch and size. There is very little room to work (1 1/8") and impossible to see if the 2 holes are lined up. I even over sized the hole in the core and still couldn't get it to align. I finally went and purchased some non expanding high heat silicone and glued it in. If you don't use the screw or adhesive it will eventually blow out due to back pressure.
I enjoy the powercore4 for my KTM 990 SMR (street bike) but found them way to loud for my taste and for the MIBS; so I purchased these dB-killers; fit is garbage, baffles aren't touching all around the cone cap and each of them was delivered with a different screw.! But, God now the sound is exactly what I hopped for: much quieter than equiped with their stock sparks arrestors but keeping its enjoying bass groove; it is now just a few dB louder (maybe 3 or 4) than my stock KTM exhausts wich are catalised. Ice on the cake: my bike is smoother at low rpm than with the stock ktm exhauts and almost (if as much) as powerfull on the global rpm range than the powercore4 with spark-arrestors. I'm very happy!