[Bex]

So, Jerry (xxx), in a nutshell don't mess with it? No gain or is it just a little and not worth the risk? I do the same high altitude, only in the dirt, and would like a small gain at no cost....no free lunch eh

See Rhino and Fordem's posts in the above link. These cars are basically factory timed according to the gas that was intended for use, ie, regular, low octane. If, as Rhino says, the timing on these cars has been set to 89 RON, the conversion to AKI (rating used in the US) is usually 4-5 points lower than that, or 84/85 octane on a US pump. So, actually, the gas you are getting in the US is a higher octane than what the car is actually timed for. The higher the octane you are using, the more you can advance your timing - gas not only ignites from spark, but also from compression. The higher the octane, the more the gas resists ignition.

 

[Bex]

I don't have an equation for timing advance vs elevation, but certainly there are posters here that have advanced their timing appreciably. Advancing to 8* is no problem. More info here:
http://www.suzuki-forums.com/suzuki-sidekick-escudo-vitara-geo-tracker/54562-timing-advance.html