Keep in mind that the LS is Luxury and Performance. Running premium fuel is recommended because that is how the car was designed for maximum stock performance. Spark Plug Rating, Timing, Etc..
Not running premium will !Removed! performance slightly allowing carbon build up, pinging, etc because lower octances do not burn as hot.
Driving habits will affect your MPG more than the octane you put in your car. Quick Accelleration from stop lights, Getting thrown back in your seat as that passing gear kicks in on the highway as you pass someone who is going to slow on that two lane highway, etc.
I guess its like having a Corvette or Viper and asking if you can run regular fuel..
Using the lower octane will make you push harder on your pedal because you will be thinking, where in the heck is my LS power that I am used to..LOL
I for one love the power from my LS and would not sacrifce that by putting in lower grade or octane fuel. Some states put Ethanol as an additive (Colorado for one) which has rumoured to dry out gaskets and other rubber engine components, so I try to stay away from the Ethanol products.