Yup, StP vibration dampers start working from 25% coverage on-wards. 50%-75% is sufficient for most applications. If you are installing a sound insulator, that needs 100% coverage.
You would only need to cover 100% with vibration dampers, if you also want to maximize the noise blocking properties. But its best to actually use vibration dampers and sound insulators together. Mid-bass would be affected by 2 things here. One is baffle design (allows the mid to breath better), and one is your damping. Damping affects the mid-bass the most. With correct baffles and damping, you should have almost a 30% increase in mid-bass and increased power handling for the mids also.