  The annihilation of dark matter (DM) in the Galaxy could produce specific
imprints on the spectra of antimatter species in Galactic cosmic rays, which
could be detected by upcoming experiments such as PAMELA and AMS02. Recent
studies show that the presence of substructures can enhance the annihilation
signal by a "boost factor" that not only depends on energy, but that is
intrinsically a statistical property of the distribution of DM substructures
inside the Milky Way. We investigate a scenario in which substructures consist
of $\sim 100$ "mini-spikes" around intermediate-mass black holes. Focusing on
primary positrons and antiprotons, we find large boost factors, up to a few
thousand, that exhibit a large variance at high energy in the case of positrons
and at low energy in the case of antiprotons. As a consequence, an estimate of
the DM particle mass based on the observed cut-off in the positron spectrum
could lead to a substantial underestimate of its actual value.
