and the Optimum Single Engine speed (green-dot) being higher for the NEO is understandable. While rated at the same 27.5k, the higher drag of the failed engine with a larger diameter could suggest the reason behind the increase of both the speeds.
GD speed for CFM Neo or Ceo is same i.e. five kts higher than IAE. To have higher GD speed the drag has to be less and not more. I have a graph by airbus thrust against altitude CFM Vs IAE in that it is mentioned CFM gives higher thrust at higher altitude while IAE gives at lower altitude. That is why on approach IAE may be requiring the extra flap setting for higher drag to keep the thrust out of idle. During taxiing also IAE tends to run as compared to CFM. On the higher level CFM OEI ceiling is four to five thousand feet higher than IAE. Also there is a distinct flattening of the one engine ceiling curves around 25000ft even at various ISA deviations.