I have talked to a few tuners some say altitude doesn't matter and the ecu will adjust for it and other say absolutely you must tune for it. So I would like to know what the correct answer is? I live almost 6k feet above sea level and most C5's run high 13's low 14's at the track (DA is around 6-8k) so the no air is killing us. The car felt like a animal in Texas but up here it feels like a slug. The best pump octane we have is 91.

 

The answer is forced induction!

Unlike the old carburetor days, your car has sensors for air temp and air density along with instantaneous adjustment based on the exhaust gas sensors. With a proper calibration any change in altitude or barometric changes will be compensated for.

 

Your car will never run as strong in the higher altitudes, no matter how good the tune is.

You simply have less oxygen to work with, a very necessary part of the combustion process, and no amount of PCM finesse will give you that, or your seal level power back.

If your current tune is stock, you should be able to lean it out some, which will help with the crap air, but this is not something I'd experiment with via mail order.
And also, if tuned to perfection there, beware the day you bring it off the mountain, it might run a little TOO lean.

 

I get what your saying now thank for the insight guys.