The annihilation of dark matter in the Galaxy could be signed as specific imprints on spectra of antimatter species in Galactic cosmic rays. Many studies are devoted to this topic, and in that context, an enhancement of the signal is often introduced and referred to as the so-called boost factor. Recent studies show that not only this factor does depend on energy, but it is intrinsically a statistical property of the distribution of DM substructures inside the Milky Way. Therefore a large variance affects the prediction of the signal enhancement due to the clumpiness of the DM halo. We investigate a scenario in which substructures consist in a relatively small number of mini-spikes formed around intermediate-mass black holes, and find boosts of order a few thousand, with a high dispersion.