Random question... I was talking to some people yesterday and mentioned I wanted to put a new exhaust and air filter on my bike.  I brought up re-jetting and one of them mentioned that since I'm at ~5,800ft I probably wouldn't need to rejet since stock it's jetted for sea level.

I'm not certain, but I'm guessing my bike is still jetted for sea level.  so i guess the question is what kind of factor does elevation play in jetting? 

The higher the altitude the less dense the air, therefore the less power the bike (or car) will make. Altitude effects fuel economy and volumetric efficiency also.

I'm about 1500 feet, but gee you're high up at 5800! At 5800 ft you're losing about 17-20% pressure, and I think that should make a considerable difference to the bike.

Because there is less air at 5800 ft, you would be running richer than at sea level as the carbs are pumping in the same amount of fuel (maybe up to 20% - not sure if the comparison is direct), and therefore yes, you may not need to rejet and an exhaust/filter may make the bike lean out and run at the ideal air/fuel ratio, improve performance and fuel efficiency.

This is possibly why I am slightly under-jetted compared to what others do (142.5 mains V the common 147.5).

Never had a problem with 6500 feet on either my 97 GS or 02 GS, no difference in performance.  Both had oem jets.  I live at about 600 feet.

You have 2 options option 1 is to just install the parts and give the bike a quick test ride followed by a plug chop that will tell you if you lean or rich. Option 2 buy an a/f meter and get a reading for your carb with out the mods than with and use that instead of the plugs.