Actual elevation isn't as important as
density altitude (DA), which I think is what you're asking about.
http://wahiduddin.net/calc/calc_da.htm
DA is calculated from your elevation, temperature & humidity and the reading it gives you is how dense the air is compared to a "standard" elevation. Air density has an important role in horsepower, because the denser the air, the more oxygen in it, and the more horsepower you'll get.
All modern dynos that I'm familiar with take the DA into account when providing the HP readout, called a correction factor, so that dyno readings on different days (with different weather and hence, different DAs) can be accurately compared.
When you're not on a dyno, though, DA has a big effect on things like at the drag strip. A lot of test-n-tuners going for lowest ET will wait until later at night for the air to cool and the DA to go down (to a lower altitude) so there's more O2 and a faster time.
FI (forced induction) engines with superchargers or turbos are more subject to HP fluctuations from DA than NA (naturally aspirated) engines. In fact, supercharged engines drove WW2 fighter planes and let them operate at higher altitudes than NA engines because they were able to grab more oxygen in the thinner air higher up. Down here on the ground though, more oxygen means faster car, so you're always looking for the lowest DA you can get. If you're a stock NA Viper (or any other car) though, DA isn't as much of a factor, as the engine itself can detect the extra oxygen and adjust for it. In fact, your engine is always adjusting for the amount of air it's taking in and the O2 it has available.
Record setting timeslips are usually in the negative DA, in fact (below sea level), even if the racetrack is several hundred feet above sea level physically, on a cold day the DA can get to -500 or -1000 feet.
As far as a direct formula for correlating DA to HP, the above quoted "1 percent Change in power for every 10 degrees." is about as good a rule of thumb as any I've ever seen. There's lots of those around the drag strip.
