That's not entirely true, Ford bases their hp figures on 91 octane, if you run less, you get less. Back when the coyotes first came out, they were rated at 412 hp in the mustang running 91 octane, but were downgraded to 395 hp on 87. So if you want the full 435 hp, you need to run at least 91 octane.
I have the Ford racing #2 power pack installed on my car and it requires 91+ octane. They even give you a sticker for the gas door to remind you. Having said that, I only run 93 octane from either Shell or Sunoco. I won't buy gas from anyone else.