I personally tend to run regular when it's nice and supreme when it gets closer to freezing. The supreme seems to pay off when it's colder - quicker warm-ups, better mileage, and no need for gas line antifreeze.... unless you're in New Mexico where you just don't need gas line antifreeze .... <;-D
I found that full synthetic oil helps and my K&N air filtermade a difference too.
But the biggest difference I notice is when I put injector cleaner in my tank every so often. The following couple of tanks are noticably better.
... the ethanol acts kind of like an injector cleaner - I'm told.
But while the injector cleaner can be helpful to sensors - the ethanol is not supported by all manfucturers' warranties.

Not that I think anyone here is worried about their warranty. <img src="/forums/images/graemlins/smile.gif" alt="" />