It depends on what you mean by "silenced". A supersonic round does make a sonic boom as it goes down range, but as a shooter the majority of the sound you hear is the gas as it exits the barrel. Adding a suppressor catches that gas and quiets that part of the equation.
Take a 9mm pistol:
Unsuppressed ~ 160db
Suppressed 115gr (supersonic) ~ 135dB
Suppressed 147gt (subsonic) ~ 128dB
A .300 Blk AR will give you somewhat similar numbers. A .223 AR will be around 140dB with a suppressor and standard supersonic ammo. No matter what, a supersonic round is going to meter around the 130db + range.
So you do get a substantial reduction in sound, even using supersonic ammo. Keep in mind "hearing safe" is 140 dB, and I consider "comfortable" to be the low 130dB's or below.
Suppressing a weapon shooting a supersonic round also pushes the sound to a higher frequency than either unsuppressed or suppressed/subsonic.
So, to wrap it all up: from a shooters perspective, you get a significantly lower sound level making it more comfortable to shoot (there are also accuracy, velocity, and recoil advantages if you want to get into that level of detail).
As a poacher, it lowers the overall sound signature of your shot, can make it difficult for others at a distance to locate you, and can sometimes make your shots sound like something other than a gunshot.