While we're here I haven't heard anyone remind us that a teaspoon of 87 octane gas has no more energy stored in it than a teaspoon of 100 octane gas. In fact the 100 octane is harder to ignite, but THAT is what allows the tuner to add ignition advance, and use aggressive mixtures, plus a builder can add higher compression and the motor won't knock. These are the things that add the power, not the energy content of the fuel.
... and these are the things everybody just skips over.
If you build a Gene Berg Approved 7:1 compression ratio engine, running 93 octane gets you less than nothing. Running E-free gas might get you a teensy bit more than E10 (because there's more energy in the fuel), but I'd bet a popsicle it'd never be enough to notice on a butt-dyno. The advantage to E-free gas is that it is less hygroscopic and corrosive - full stop. This makes it "better" (if not rebuilding carbs so often is an important consideration) in a carbureted engine, but makes almost no difference in an EFI engine.
More power from any 4-stroke (cycle) engine comes as a result of optimizing one of the "cycles": "suck, squeeze, bang, blow" (intake, compression, power, exhaust) . Better flowing heads allow better cylinder filling (and cylinder evacuation to a lesser extent) - better "suck". Better ignition timing and spark, along with a well designed combustion chamber (again with the heads) provide a more complete ignition cycle - a better "bang". A better exhaust with some extraction occurring (because of the Bernoulli effect) provides better cylinder scavenging - a better "blow".
It's during the compression ("squeeze") and ignition/power ("bang") cycles that the type of fuel comes into play. Obviously, the intake cycle determines the total potential energy of the mixture in the cylinder (by dint of how effectively the cylinder was filled) - but it is the compression and ignition/power cycles that determine how much of that potential is converted to kinetic energy - how much power contained in the fuel is actually released. ICE 4-strokes are notoriously bad at releasing the potential energy entrained in the fuel. Converting as much of the potential energy in the fuel into kinetic energy at the wheels as possible is how an engine is optimized.
Having more potential (higher total energy content in E-free vs. E10 gas) energy means nothing if it is not effectively converted to actual (kinetic) energy - more commonly referred to as power.
Better cylinder filling in the intake cycle and more compression (in the "squeeze" cycle) is the magic elixir here - the more mixture you can compress, the more energy will be released once that mixture ignites. More compression leads to a more forceful power ("bang") cycle. Every time.
The limitation is in how much you can compress the mixture without having it ignite spontaneously without any external spark. This is uncontrolled ignition - "preignition", as it is called. It's very, very bad. It can (and does!) break important, expensive things deep in the innards of your engine. It blows holes in pistons. It breaks connecting rods. It is to be avoided at all costs.
Thar be dragons thar.
Higher octane fuel is harder to light and provides a resistance to this preignition. Higher compression is free power, but a fuel with a progressively greater resistance to preignition is required as compression is raised. The practical limitation of this on the street is what will work with 93 octane (91 in Kalifornia). Some places have 100+ octane race gas, but running that means never leaving home, which is fine if you are racing or are looking to cruise around your hometown and nothing more (ever).
Running higher compression and higher octane fuel means needing a more powerful spark to ignite it. Remember, higher octane fuel has a greater resistance to preignition, but it also has a higher resistance to controlled ignition by means of the spark plug. That 009/blue coil might not be enough. It is almost certainly not accurate enough to provide a spark at the optimal time every time.
In addition to raising the octane (by nature of being less volatile), the alcohol in E10 cools the incoming mixture (charge) by means of absorbing an enormous amount of heat as it changes state from a liquid to a vapor. This is called the latent heat of evaporation, and it's what makes your A/C work.
This is a very good thing in a hot running air-cooled engine, and it's called "charge cooling". A cooler charge allows more density in the intake cycle, and more complete cylinder filling. This is a really nice bonus afforded by ethanol in fuel.
We have the idea that there are explosions in the cylinder every ignition or power cycle - but hopefully there are not. What we are looking for is a quick controlled burn - a flame front radiating out from the spark-plug across the top of the piston, burning the mixture completely. How well this works is a function of combustion chamber design, and this is what makes Pat Downs an order of magnitude smarter than Stan Galat. A good head porter understands combustion chamber design.
If the burn is incomplete, there is a loss of power. When it is uncontrolled, there is the potential for catastrophic damage. In extreme instances, higher octane fuel in a low compression engine with poorly designed chambers will not completely burn the mixture (charge), which means there is power lost. In every instance, running lower octane fuel in a high compression engine is an invitation to catastrophe.
Modern turbocharged high-compression engines designed to run 87 octane fuel are able to do it by means of really great cylinder head design, variable valve timing, super-precise mixture and spark-curve control... and with knock sensors backing everything off if things start getting out of hand.
I bought a 2.7L EcoBoost F150 a couple of weeks ago. It's nearly a miracle, burning 87 octane while running 18 lbs of boost, and making 400 lb/ft of torque at 2000 RPM from an engine not very much bigger than a 2332 Type 1. Such is the state of the art in ICE engines - it's amazing. All I can say is that in addition to some mechanical wizardry, the pointy-heads at FoMoCo are relying pretty heavily on the knock sensors to keep this thing from grenading. It's got a 100k mi powertrain warranty, so they must be pretty confident. I'd be less so if I were sending such a device out to be used by the great unwashed.
The holy grail is running the lowest octane you can with the setup you've got. It's also really dangerous, and if your builder doesn't have the resources of Ford Motor Company, I'd err on the side of caution. Nothing is gained by running higher octane fuel in an engine not built with higher compression to utilize it, but absolutely everything can be lost by running too-low octane in a high-compression engine with no knock sensors.
I have no idea what is needed in an EJ25 with bumped up compression, bigger cams, and a non-stock ECU (which doesn't utilize a knock sensor), but I'd rather be safe than sorry. E10 93 is available everywhere besides the mountain west and the left coast.
You do you, @550 Phil, but I'd stick with what was working.
