Armchair developer speculation time...
If you were sitting on this property, would you wait a few more months until Amazon announced before moving forward with plans? I know I would. I'd cover myself in relish and mustard. I'd be delicious.
If HQ2 announced they were coming, I'd go back and find a way to push for every inch of height and extra unit I could. If they go elsewhere, proceed with what they've already got.