I have not tried it on my G-Stock Subie, but have experience doing it on my hot rod Yugo, and dozens of hot rod Ford/Saab V4 engines.
The Yugo has dual carbs, and I lived at 5,000+ ft elevations, but raced at sea level conditions. Indexing the plugs helped when the car was a bit too rich (too much fuel), so I didn't have to change jets too often.
It wasn't anything you can feel, just helps to ensure a more even burn.
I haven't really ever seen a write-up on it, but from what I have heard people with big old american trucks do/did this... cut the end of the ground electrode then gap the thing so the spark fires from the center to the side, rather than straight up. That would also give you more of a direct shot to the center of the combustion chamber.
Oddly enough they say it cuts the spark plug life drasticly.
If you put washers (like that article was speaking of) on the spark plug aren't you running the risk of screwing things up? I mean if the electrode isn't far enough down into the chamber it's not going to work very well.