Many people at Op Gun Store 2 at the mill last weekend saw my Echo 1 P90 and heard it with the supressor. And all were impressed at how silent it was. Even the motor in that gun is fairly quiet. Then they were shocked to hear how loud this fucker is without the supressor. That aluminum piston head really makes quite a racket.
So yes, an airsoft supressor can make a HUGE difference, providing of course that it is foam-lined and the inner barrel doesn't run into it.