We measure the net energy efficiency of supernova (SN) and stellar wind feedback in the starburst galaxy M82 and the degree of mass loading of the hot gas piston driving its superwind by comparing a large suite of one and two-dimensional hydrodynamical models to a set of observational constraints derived from hard X-ray observations of the starburst region (the fluxes of the Heα and Lyα-like lines of S, Ar, Ca, and Fe, along with the total diffuse E = 2-8 keV X-ray luminosity). These are the first direct measurements of the feedback efficiency and mass loading of SN-heated and enriched plasma in a starburst galaxy. We consider a broad range of plausible parameters for the M82 starburst, varying the age and mode of star formation, the starburst region size and geometry, and SN metal yields. Over all these varied input parameters all the models that satisfy the existing observational constraints have medium to high thermalization efficiencies (30% ≤ ≤ 100%) and the volume-filling wind fluid that flows out of the starburst region is only mildly centrally mass loaded (1.0 ≤ β ≤ 2.8). These results imply a temperature of the plasma within the starburst region in the range 30-80 million Kelvin, a mass-flow rate of the wind fluid out of the starburst region of and a terminal velocity of the wind in the range v _∞ = 1410 – 2240 km s^–1 . This velocity is considerably larger than the escape velocity from M82 (v _esc 460 km s^–1 ) and the velocity of the Hα emitting clumps and filaments within M82's wind (v _Hα ~ 600 km s^–1 ). Drawing on these results we provide a prescription for implementing starburst-driven superwinds in cosmological models of galaxy formation and evolution that more accurately represents the energetics of the hot metal-enriched phases than the existing recipes do.