A silencer won't do anything to improve accuracy. All a silencer is going to do is muffle the sound of the bb ejecting from the rifle, since a sniper rifle doesn't use any sort of gears or motor, the only sound heard is the pressure of the air when shot and the actual sound of the bb releasing from the gun, by placing a silencer on there, you cut your sound down aprox 75%. Since your the only one who is going to hear your gun actually shoot you probably won't notice a difference. But with Livonia's rifle he took a shot on me (I watched it through my scope, hit me in the lower left face, thank god for full face bush hat) but I didn't hear anything. I was maybe 50 yards away at that time.
But even then, when your any closer if your going with a high end sniper rifle all you'll hear is the whiz of the bb going past your head if not you won't hear anything. But to answer your question in short, a silencer won't do anything for accuracy.
Sorry for the long post, I just felt like typing.