I've been thinking about the height of this building. I recently emailed Shonda Novak at the American-Statesman. She does most of the development related articles. I was asking her if she'd heard anything official about the height. She said all she's heard is 45 to 50 floors.
I was thinking about Los Angeles' Ritz-Carlton Hotel that is part of their convention center. It is also designed by Gensler, as Austin's new hotel will be. That hotel is 663 feet tall with 54 floors. I figured the floor-to-floor heights should be similar for both hotels. So taking the numbers from LA's hotel, it is 663 feet with 54 floors.
So 663 feet /54 floors =12.2777778 feet per floor.
So if it's 45 floors, it would still be pretty tall.
12.2777778*45 floors=552 feet to the roof. The spire seems to be between 150 and 200 feet tall, so even if it's only 150 feet tall, the building will still be 702 feet tall.
And if it turns out that it is 51 floors as the original rendering suggestions, it'll be considerably taller.
12.2777778*51 floors = 626 feet to the roof + a 150 foot spire = 776 feet
Someone on the forum mentioned they thought the spire might be as tall as 180 feet. If the building ends up being 626 feet to the roof as my numbers above indicate, then it could be 806 feet to the spire.
By the way, 626 feet to the roof would put it only 4 feet lower than the main roof of the Austonian.
And here is the construction thread for the LA hotel. It's a really nice building, and was the tallest building LA has seen since the early 90s. Notice that the building hosts two different hotel chains. A Ritz-Carlton and a JW Marriott. One of the articles for the Austin hotel mentioned it will have two hotels in the building. So this LA hotel is a good hint at what we might see.
http://forum.skyscraperpage.com/showthread.php?t=129618