At 251t Flap 20 or 25, its 160 plus, the 300 is more like 150 kt, its 13 t lighter on landing.
As a result of flight testing the A380-800 has come in under its predicted Vref, how do you know what a A380-900 will do ?
Unless Airbus changes the wing on the A389 and/or the A389 weighs the same as the A388, its approach speed will increase relative to the A388.
Hint: If the A389 is to have a greater payload than the A388, its max. landing wt. must be greater than the A388 to carry the greater payload and the associated structural wt. increase.
As far as the 773ER approach speed is concerned, why are you quoting a speed associated with alternate landing flap (F25) or non-normal landing flap (F20)? At the prime landing flap position (F30), the MLW Vref is 150 kt.