The last time they went to the C of A to boost the tower up to 75 storeys was so that they could get the permitted density increased. This plan has a larger floor plate than the Bazis plan did, so they had essentially maxed out their density at 70 storeys.
The rumours of another increase would be better described as wishful thinking on the part of skyscraper fanboys. They have essentially been fuelled by the fact that, yes, the building is already allowed to go higher than they are going, but the minor reconfigurations to the building's design that you are hoping would be enough for it to take advantage of the additional height allowed just aren't there to be made. The units are sold and the floor plate is decided upon. Any further increase in height would have to come from the C of A granting more density.
Finally, to re-emphasize this, it's not Great Gulf that has been fuelling rumours of another increase.
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