At around 12 c/s and higher (I have someone with 25 c/s) pawns become invisible and all particles they generate (via shooting) vanish and have no effect. During that console errors are generated. He can work and operate normally though. I use hardcore modpack, but I don't think it matters in this case.
Some screenshots:
Circumstances:

Effect:

Console:
Some screenshots:
Circumstances:

Effect:

Console:
(because of weaker construction),
)
.