We have compared the acceleration of high-energy ions from the rear surface of thin foils for various prepulse contrast conditions of the ultraintense laser pulse irradiating the targets. The experiments were performed using the Laboratoire pour I'Utilisation des Lasers Intenses 100-TW laser facility with 15-20 J energy pulses of >1018 W/cm2 intensity irradiating aluminum targets of variable thicknesses. The laser pulse contrast ratio ahead of the main pulse (prepulse) was varied using either a fast Pockels cell or a single or double plasma mirror (PM). The latter was installed at an intermediate field position, in between the focusing optics and the target. This way, the effect of reducing the laser prepulse by the PM was optimized. With both methods improving the laser pulse contrast, we have observed that one can significantly reduce the thickness of the target used for proton acceleration and, at the same time, increase 1) the maximum energy of the accelerated protons and 2) the energy conversion efficiency of the process. This offers interesting perspectives from high-energy (5-10 MeV) ion acceleration using sub-petawatt laser facilities. A full characterization of contrast enhancement due to the PM, as well as a detailed analysis of the accelerated proton beams, is presented.