A properly set up string silencer will not cost much, if any speed. Might even gain a few FPS if you get it in the right spot.

And there is absolutely no way an STS is more quiet than good string silencers. It is physically impossible for something to slam into something else and make no noise. I haven't heard any of the dampner designs that are remotely quiet in my opinion.

I bet they do help with recoil though. The again so will limb savers.

Paul