I'm power level 282 at max and none of my engrams drop anywhere near that. All my rares and purples drop at 273. No higher or lower. The only way I can get higher is with exotics.
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That actually seems typical of destiny pre-raid release. There has been in the past a soft cap where only certain activities will pass above that cap. Exotics used to have a soft cap higher than legendaries so it would stand to reason that this would still be in place. Even so though those exotics shouldn't hit the 300 light limit I would imagine they will cap at 290.