I haven't noticed any difference in engine performance. I often travel to rural areas where oxygenated fuel isn't required by the EPA, so I get to switch back and forth between fuels, and I don't notice any difference at all. I've also tried higher octane fuels (100 and 105) at the track and haven't noticed any difference with them vs the usual 92-93 octane, either.
What I HAVEN'T done is use anything less than premium fuel, though.