Your test is similar to Ron Laughlin's testing - he simulates about 1200fps with BH209.
Ron's test have shown many bullets failing at lower velocities. One performing factor is the large HP required to open a bullet at low velocities such as this - regardless of material.
I believe you are promoting the silencer by your sub-sonic Velocity test, unfortunately with that light bullet you're only at 400 ft-lbs of energy.
I think you could actually kill something sub-sonic with a bullet weight around 500gr at 880fps because the energy is equal to this velocity - nearly even out to 100yds.
Good luck and keep us posted.