There's only so much you can do to a bow, the big speed gains are poundage and arrow weight and it sounds like you are already done what you can with those.

You mention silencers, these will slow down the bow 5-10 fps depending on the type, sounds like you have felt ones, which I believe slow it down the most.

The "thing" behind your nock, is it metal? If so, replace it with a string loop, that will add a couple fps, or even just remove it, the metal ones are heavy/slow.

--Bob