I believe there are two or three different CAIs that don't require a tune, and I infer from that they don't require octane ratings higher than 87. As little gasoline as I use (ten gallons a month or so, when not on a road trip), the price difference between 87 and higher grades isn't significant for me.
The K&N "Typhoon" intake has been CARB approved, and claims about a 9 HP improvement over stock, without a tune. I believe the 2010 15 HP includes the effect of a tune. It'll be similar, if not the same, as the"Bullitt" intake/tune combo, which is available now from Ford Racing Performance Parts, and stays within warranty parameters.
Aftermarket tunes improve not only power output, but drivability. Better throttle respnse is quite noticeable.
I'd recommend going to allfordmustangs.com and searching for your interests by keyword. Forum members there are knowledgeable and generous, and you can get useful answers to nearly every question.