Most likely,
  • iLevel progression of normal gear obtained from duties 51-60, follows from where it had stopped at 1-50
  • whatever the max iLevel at 50 may be, greater iLevels than that will have lv.60 requirement
  • Actual raiding gear will be good for all your leveling from 51 to 60
  • All the discardable green/grey gear from 51+, is useful for the players who starts from scratch and will go level to 60, without the "pause" at 50 we actually have. They will start equip greater iLevels when starting the 60 endgame.

More unlikely:
  • Actual endgame content scaled upper to lv.60
  • i80+ as normal drops on 51+ regular duties