my opinion is that pillaging a farm on a hostile tile should equal to resting a full turn on a friendly tile.
in a matter of supply, you already get full supply in city, 75% supply in friendly tile, 50% supply in neutral tile and 25% supply in hostile tile.
troops are "healed" due to availlability of supply so pillaging could bring 50% more supply resulting in healing 15 hitpoint instead of 5 hitpoints.
the cboost of health would be less important and yet quite usefull if well placed but could need some few other things to be change to balance it, like removing the "wall give automatically an army of the best range unit to the city for free".
if healing in hostile tile became more difficult war would need more preparation, tactic & strategy would be needed more and wall could be used to simply "stall the enemy" allowing you to bring some reinforcement from nearby cities.