I haven't see this posted, but seeing that GM actually confirms using 91 octane is better I thought I would link it.
2011 Chevrolet Cruze: Hot Weather MPG Part 2
The original link is hard to follow. I followed it and found this after a few minutes. -Mike.
Link to Edmonds report. Note the fourth paragraph:Instead, the hot weather led to a very noticeable lack of drive-away power and sub-par highway fuel economy. Subsequent discussions with GM powertrain bigwigs and our own Jay Kavanagh revealed that small turbo engines are especially octane sensitive, which means their computers may agressively dial back the engine calibration in order to ward off knock in high load situations or in very hot weather.