The closer you get to the center of the string with any weight the more speed you lose. However there is usually a sweet spot that will actually gain you a few fps. That is why most new bows come with speed nock sets on them. With a single cam it will be on the bottom, normally a few inches up from the cam. On a dual cam bow you would most likely need to place them on the top and the bottom.

You would need a good chrono to find the right place to put them.

So the closer to the axle's you place your dampners the less speed you would lose. You just need to experiment to see where they are the most effective at reducing noise and vibration. I haven't seen many string silencers that cost you a large amount of speed though. Not enought that I would consider not using them. The less vibration in the bow the better. I even use them for target shooting. Why have my string vibrate more then it needs to.

Paul