My '97 SE (L36) most definitely runs better and gets better mileage using 89. It currently has 170,000 of that octane on it and is running smooth as a top.
That is one example of why it is best to avoid very generalized statements unless you have data or other supportable facts to back them up.
A fair generalized statement might be, "Most cars today are designed to run their most efficient on 87 octane and most owners will not see any gain from using higher octane gas. In fact, since higher octane gas is harder to ignite, using higher octane gas when not needed CAN result in improper burning, leading to increased deposits in the combustion and exhaust systems."
There will be some that argue that those of us who use higher octane than required are simply masking issues in our cars that should be mechanically fixed instead of "covered up" by using other than rated octane gas, we would likely respond by indicating that by using the proper heat range plugs to take advantage of the higher octane we are not harming, but actually being kinder to our engines.
In addition, it is well known that KR or pre-ignition is a seriously harmful problem. A properly maintained engine that still experiences "pinging" under load will often benefit from one grade higher octane. Indeed, as has been shown a number of times both here and on other forums in these cases, a higher octane yields much better mileage, therefore effectively paying for the increase in price.
OK, I'm done...
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s