The 32% is a full round trip from battery->kinetic->potential->kinetic->battery. Starting with the potential energy as we did above you cut half of that out, so if you assume the driving side is as efficient as the regeneration side, we'd expect to see sqrt(0.32) or 57% of the potential energy recovered into the battery on the regen side.
Remember too that that battery ends up heated anyway. The energy to do that is from one of our two energy sources - the potential energy released as the car's mass descends in altitude, and the battery's energy. This is the case whether or not we pre-heat it - the only additional losses from pre-heating is whatever extra heat is lost by the warmer battery between pre-heating and driving.
So the question ends up - do we lose more energy from having a warmer battery earlier, or from having to use the brakes more on the way down?