I meant nocking point/arrow nock. It sounds like that fixed it if they're not tailing though.
As for the slightly slower speed, I can't tell the difference unless I am shooting from 40 yds or so, where I can notice the arrows dropping more than expected. I wouldn't worry about it. Just be consistent.
The effect of silencer position on spine is real, however. Especially when using a bow matched to shafts that are right on the border of being too weak. Often moving the silencers to the string nocks will indicate you got a batch on the low side. With some batches it makes enough of a difference that I always pull out a bow square out at the truck and check my silencers right after checking my brace height.
Actually, each year when satisfied with setup/tuning, I even record silencer position right along with my brace height, arrow length, string length, point weight, and point-on distance.