The top of 50 First Street is actually more of an elongated hexagon shape in plan. This can be seen if one looks more closely at the rendering.
Perhaps more height can be added as long as it does not add significant shadows. For now, we only know of the possibility of some sort of added art work at the top of Salesforce Tower. Such artwork should be clearly visible from several miles away.
Source: San Francisco Planning
By the way, Seattle's talks for what could be the new tallest American tower outside of Chicago and New York is not yet a solid proposal.