The rule of thumb that I have read is 3% reduction in horsepower for every 1,000 ft in altitude.
Sea Level - 10hp
1,000 - 9.7hp
2,000 - 9.4hp
3,000 - 9.1hp
4,000 - 8.8hp
5,000 - 8.5hp
6,000 - 8.2hp
...etc.
I'm sure the differences could be measured, but not sure where someone would notice a...