You WILL receive better performance and economy results from the higher octane levels, that is just the nature of the octane battles. However, even an 87 octane performance and economy tune will out perform the stock tuning. The difference between the econ and perf tuning is so impressive that my wife driving the truck after switching from the perf to the econ tune asked what happened to it. She said something had to be wrong because it felt gutless. I explained it to her and she requested that I load the perf tune in there and leave it. This coming from a woman that has no desire to be involved in anything performance oriented...lol
Now, that being said. Here is something that you CAN do for testing tuning and having all octane levels available for your testing. When you place your order, ask for 87 and 91 octane tunes. You see, whether you have an X3, X4 or Livewire, when you are loading a tune, you have the option to adjust Global Spark. If you load an 87 octane tune with the Global Spark increased by a +3 degrees, that basically makes that 87 octane tune act like an 89 octane tune. Same with the higher levels. So if you order 87 and 91 octane tunes, you can have the option of running all octane levels if you so choose.
The thing to keep in mind when changing tunes is that it takes some miles/time for the truck to properly relearn the shift points, etc. So, for the scenario that includes hauling a boat to the lake, dropping it off, taking the highway to the dunes and then hammering on the dunes. You do NOT want to be changing the tune at each change without making sure you have some drive time between them. For example, if you are running an economy tune for everything else and you make it to the dunes and load your performance tune, you then begin to immediately hammer on it at wide open throttle, when you leave, if you do so without changing the tune again, driving normal, you will most likely have some hard shifting, shifts that act like they kind of want to hang so to speak, etc. Likewise, when changing from a perf tune to an economy tune, it takes a little while for the true benefits of the economy tune to be experienced. The more you drive it on that tune, the better mileage you will get; to a point (or until it levels out).
As for warranty, no. If you want to know for certain that you will not affect your warranty, DON'T tune it. SOME Chevy guys in the diesel world have tried to keep a stock tuned ECM on hand, but there is a flaw in that theory. The dealership can look at the data logs stored in the ECM and tell how long a tune has been run, how many drive cycles the truck has been through, etc. If there is anything out of the ordinary, they can and in most cases will void your warranty and in doing so they will flag your VIN with Ford to ensure you aren't able to sneak off to another more mod friendly dealership. The technology has become so advanced that the dealerships are now able to tell if someone has modded their truck, then put everything back to stock to bring it in for service work. Granted, it takes a real jacka** dealer to go to that extreme, but there are plenty of them out there like that.
Also, for those wanting tuning, etc but also thinking of an exhaust. For the most part, a catback exhaust does not affect the tuning enough to matter. So installing the tuning first and then installing a catback after that would not warrant retuning. Header installation on the other hand does have a major effect on tuning and performance and should be retuned. Cold air intakes have more affect on tuning than catback exhausts and while they do not have to be tuned for, the tuning CAN dial in the performance for the intake to make it worth better results overall.
Octane levels in your area should be a major consideration in your choice of tuning. Around here, you can hardly find 91 octane, 93 is our Premium level. But like most have said, their areas offer mainly 91 octane.