if low is useless in slippery conditions then selecting MED will have very little effect on the landing distance.
LOW and MED give a preset rate of deceleration. If low is unable to achieve the target rate of deceleration due to a slippery runway selecting MED or even max manual braking won’t increase the rate of deceleration. Increased braking effort will only make a difference if you pass over patches of grip and if that’s only on one side you run the risk of a rapid yaw and then potentially going off the side.
Spoilers and reverse thrust are your friends on slippery runways as they’re symmetrical.
LD