I have the smaller one which i run on my NGH 17cc.
It definitely takes the harshness out of the exhaust note and gives it a more bass'y tone. I don't know if it reduces the overall dB level by much at all, bit it's certainly more pleasurable when standing close to the engine.
Loosely fill some coarse steel wool in your muffler rather than plugging the outlet stinger. It attenuates noise more efffiently with no real loss in power unless you cram it full. Another more effective silencing tip, install some soft silicone rubber to the stinger, about 8" long