^^ Awesome shot, above.
Regarding all the speculation around the height, floorplate sizes, taking down "the lower building", etc, here's some quick math gasoline for the fire:
2,500,000 sq ft, at 70 floors, works out to an average of about 35,714 sq. ft per floor. A perfectly square floorplate, with every floor identical for the entire building height, would have dimensions of about 189' per side. At 75 floors, you're still at 183' per side (33,333 sq. ft per floor).
I haven't walked past 270 Park in a while, but looking at that picture above, I'm guessing that's considerably bigger than the current, main tower of that building. But the entire block (ie, entire 270 building, "lower portion" and main/taller portion) would certainly support some combination of dimensions that get you to the 33-36,000/floor. (200' x 175' is 35,000 for example -- an acre is 43,560(!)).
THEN, at say 15' per floor, you're at 1050-1125 feet for 70-75 floors. At 18' per floor: 1260-1350 feet. That's just for inhabitable, working space for employees.
Keep in mind this is with all floors the same size -- that could easily not happen. And maybe 15' per floor is too conservative (18'-ish is 1VB dimensions, and this is a global headquarters -- they won't skimp). So to get to 2,500,000 sqft (room for 15,000 employees vs. 6,000 being the biggest tell), you could easily have more than 75 floors, almost certainly starting from as much block coverage as the city will approve. Then tack on non-working, "pure aesthetics" (spires, etc...) and 1500'+ doesn't sound like too much of a stretch.
Another comparative:
Bank of America's building is 1200' to the spire, 900'-ish of working space and 2,100,000 sq ft spread over 54 floors.