  We measure the net energy efficiency of supernova 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 1 and 2-dimensional
hydrodynamical models to a set of observational constraints derived from hard
X-ray observations of the starburst region (the fluxes of the He-alpha and
Ly-alpha-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 supernova heated and enriched plasma in a
starburst galaxy. All the models that satisfy the existing observational
constraints have medium to high thermalization efficiencies (30% <= epsilon <=
100$%) and the volume-filling wind fluid that flows out of the starburst region
is only mildly centrally mass loaded (1.0 <= beta <= 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 Mdot_tot ~ 1.4--3.6 Msol/yr and a terminal velocity of the wind in the range
v_infinity = 1410--2240 km/s. This velocity is considerably larger than the
escape velocity from M82 (v_esc <~ 460 km/s) and the velocity of the H-alpha
emitting clumps and filaments within M82's wind (v_H-alpha ~ 600 km/s). Drawing
on these results we provide a prescription for implementing starburst-driven
superwinds in cosmological models of galaxy formation and evolution. (Abridged)
