Raimey I believe the fact that 1200 fps and sonic are fairly close together is just coincidence.
It's a point of diminishing returns where much more velocity than that just makes recoil and noise and not much decreased time of flight or increase in energy at the target, at least with lead.
With non-tox that's less dense than lead, more MV is about all you can do to slightly improve things.
You can't fool mother nature. Round (or nearly round) projectiles are going to slow down in a big hurry since drag is a square function of velocity.
!200 fps is just a sweet spot.
