99% of the time the gas is cut with ethanol to raise the octane, or the other 1% of the time additives that have vastly lower energy content than gas. Go look at the threads about people complaining about their MPGs dropping in the winter -- that's when ethanol content in gas is increased. Even if higher octane helped, you would have to see a 10% improvement in fuel economy for it to save you any money. That's a tremendous increase in efficiency, something Ford would have considered. Hell, most manufacturers switched to 5W20 to squeak a little bit more out of the engines. Ford put in radiator shutters to get an extra 3% efficiency -- why wouldn't they want a car that pulls down 30mpg in the city cycle and 42mpg on the highway? Even if people ended up using 87 and getting 28/38 instead, it would still make for great numbers in advertisements. Look at it another way; see the people running E85 and see what kind of mileage they get out of it. E85 is right around 96 octane (RON+MON/2) -- if higher octane = better mileage then logically E85 would be what everyone is filling their cars with instead of E85 being a cheap way to skirt CAFE ratings or for people that convert their cars for the massive power gains that are available. No one ever converts for fuel economy and actually gets it.
Will it make more power? Possibly, in situations that you have the engine running under a heavy load for extended periods of time because the engine will have knock issues. The compression ratio is extremely high in these engines and even in day-to-day stuff you'll have the occasional ping/knock. If you take it to an HDPE event, premium fuel is a good idea. Same goes if you're towing or load the car down in the summer months.
Any increase in economy is going to be from being conscious about getting better MPGs and being easier on the pedal. There's also the chance of carbon deposits building up quicker, but that highly depends on what additives are being used. Some cause it, some have a negligible effect. It's too soon to tell how these engines do with carbon buildup but virtually every other direct injection motor on the market has issues so it's something to consider.