Tomshardware.com often has the power and cooling covered in the reviews. If not that site look to other reviews.
However isn't your case airflow a factor as well? I'm lost count of die hard gamers that either leave the side cover off or punch a 2 by 2 foot hole in the case sides.
I want a new gpu because im in the process of upgrading my pc and me two preferences are the Asus R9 280X and the Sapph R9 270X TriX. Which one has the best colling option?