I found more legendary engrams from lvl 2-6 enemies than from higher lvl enemies and theres no official bungie statement about those droprates. I also rush through the strike playlist sometimes because I want the guaranteed engrams to lvl up crypt and maybe get legendaries from the lvling-him-up reward. I kill many mobs while farming and doing bounties etc. As long as it is not proven and I personally dont see any difference between killing enemies with 1 hit or killing lvl 24s (which takes obviously longer) I will keep on doing strikes for the finishing rewards and not to kill-farm