I've never seen a dyno chart on this, so most accounts of improvements are possibly anecdotal, like more intake noise making the car faster. But I would think that running the factory max of 18 degrees is safe, since it's still in spec. Just be sure to use a timing light. Also be sure to keep premium in the tank, because every time premium goes over $3/gal, it's tempting to switch to mid-grade.

Does anyone seriously think there's danger of engine damage as long as it's within spec? Like maybe going up a long grade with a load and the a/c on? Just curious what the thinking is out there, because this is something that anyone can do at low or zero cost.