Think of Thermal Vent (TV) as applying to the hardpoint, not the ship. TV inverts the heat generated by a hardpoint, so if a beam generates 5 units of heat per second (which is then dispersed into the ship) without TV, with the experimental it will vent 5 units of heat per second from the ship instead. So TV not only removes the heat generation of the weapon, it inverts it completely.
Because of this mechanic, the amount of heat vented by the experimental is determined by the heat the hardpoint would normally generate. If a beam would have generated 5 units, it now vents 5 units. This means large beams vent more than smalls beams. It also means the efficient mod actually lowers the venting properties of the hardpoint (as efficient lowers the heat generation). As the experimental applies to the hardpoint, the effect will stack, with more hardpoints venting more heat (2 huge beams on my 'Vette can completely negate the heat of my SCBs).
DPS, and therefore drop-off, do not factor into the equation. Whatever heat the weapon should normally generate, is now inverted.