Lizard King I have a 2002 GT as well, build date 3/2002
and it's got about 1700mi on it now. I have run 89oct in it before, and I run 87 in it right now. I haven't noticed any pinging/knocking or performance degredation in the seat of the pants. Do you always get gas at the same place? I never get gas at Holiday here because of their "Blue Earth" gas campaign. It's possible the place you get your gas from is subpar? Also, I know you have a couple mods now. Anything that plays with the timing could be effecting your car, or possibly a defective anti knock sensor is pulling timing?
Did you have your car at the track when you were running 87? I'd be curious if it traps any higher on higher octane fuel.
Also, different cars do run differently. Take the old 5.0's as an example. In proper tune, people are able to bump the timing from 13-17* on premium gas. That's a 4* difference.
The difference on the 99+ cars is going to be obscene. Tolerances in the head are TERRIBLE on the new GTs. I've never seen anything so ridiculous in my life. I don't know who the moron working on the PI head design team was, but the tolerance is +/- 1.5cc in combustion chamber size. Same as the older heads, but the problem is the overall combustion chamber size on the PI heads is a meager 44.75cc. The pistons have a 17cc dish to them, but still. Your head's could have combustion chambers anywhere from 43.25cc to 46.25cc under Ford tolerences. Now, for the sake of argument we'll say your car is running really strong and you have at least one cylinder with a 43.25cc combustion chamber. Your compression ratio on that cylinder is going to be about 9.25:1, whereas if you have at least one combustion chamber with a max tolerance of 46.25cc, the compresion ratio will be 8.75:1. That's 1/2 point of difference between minimum and maximum compression from cylinder to cylinder. It's certainly concievable to me that a few real good factory "freaks" are going to have a lot of the 9.25:1 CR's and they might favor higher octane fuel a little more. There are also the "slow" 4.6's that will be making power based on a lot of the 8.75:1 CR combustion chambers. Of course the computer is designed to make sure the engine isn't knocking, and that means a little internal timing retard and more fuel on an engine that might be doing a little pinging. Of course, this is all grasping at straws because nobody knows what your head would truely cc out at. If your car truely runs better, and traps higher in the 1/4 under higher octane fuel, then you have a sensor malfunction, or your combo really does need the higher octane fuel to keep from pinging.
bstang46, How long have you been running high octane fuel in the car? If there is carbon build up, you can certainly run into the problem you are describing, although the anti-knock sensor doesn't seem to be working super good? If you've been running high octane fuel in the car for an extended period, the computer may base it's injection and timing curves on the higher octane fuel? I'm not sure how internally adjustable the newer computers are. I know on my 87GT, driving it hard lets the computer adjust itself to a more aggressive state of tune.