I would suggest that it is safe - after all you can dispatch with one pack inoperative and are limited to FL315 (from recollection). Whether or not is is effective, I don't know, but on the B747-400 with one pack inop, the other two packs run permanently in 'high flow' mode, and the MEL suggests that two packs in high and 3 packs in normal flow result in no discernable fuel consumption difference.