Hi Mark - yes AM does do quite a marketing job with their tunes & stuff like that - when I first started looking at it I was amazed also - I had the "I gotta get its" until I looked deeper.
I think if you look closely you will find AM does it's base tests with 87 octane gas - and final tests with 91 or 93 octane - and voila - instant hp. I think if they used the same gas either 91 or 87 start-finish in the tests you would see minimal if any overall gains. Maybe I'm wrong, Maybe am would like to chime in on this - maybe I'm not wrong. Remember the stock mustang makes different power with different octane gases - this is where most of the gains come from.
I think axlebacks + intake will do squat for you - the tune will make the difference. It has the effect of giving you maybe 5-7 hp more - but what it does do is affect other things like gas pedal lag and responsiveness - so the car will definitely feel more awake. It does change the torque/hp curve - from what I understand a tune will shift the torque curve more toward the low end of the spectrum - so you will have more off the line, but what you gain low end you typically loose top end. Nothing is free in this world, and if it sounds too good to be true it always is.
I have to agree with the torque converter and gearing issues mentioned above - a tune alone will make a big difference in how the automatic shifts - but be warned - I have heard alot of people on this forum complain that am's tune is a hit/miss with automatics - maybe it is better with their new tunes - I don't know - an automatic tranny is not something you want to mess too badly with - it is a costly replacement item.
You may be best off putting your cash with just a tune/ torque converter and some simple suspension mods like better lca's to eliminate wheel hop.
You pretty much nailed it except for the part about a stock car making more power w higher octane, but you were close. The more timing you run the more power you make and the more timing you run a higher octane is required so there won't be any detonation. Say you have a car that is "recommended" of 87 octane and you put 93 in it's still going to have the same power because the timing wasn't advanced. What is the recommended octane for the coyote anyway? Say it's 91 and a company runs it on 87 and retards the timing for a base pull, they could really fudge a test. But yeah axle backs make noise, gears throw you back in the seat harder and make you shift soone, but the "feel" far out weighs the actual time knocked off.
Looks like I picked the wrong week to quit sniffing glue.