People have a range of rules they follow on this Mike. They're all fairly close to oneanother. My understanding is that ya generally don't count the first 1000 feet. After that, according to my "rule" it's about 3% per 1000 feet.
Air Density decreases at a rate of 2.9% - 3.0% for each 1000 ft. of elevation above Sea Level.
Atmospheric Pressure's about 14.5 psi for flatlanders (14.7 at MSL)
Atmospheric Pressure @ 9000 feet = 10.5 psi
Pressure Loss = (14.5 - 10.5) = 4.0 (4.0/14.5) = 27.58 % @ 9,000 feet
An engine sucking cold air and/or with a tuned intake can overcome this a bit. Using the higher CR in a l'oid (COV closed) also helps.
Unless you're well up in the hills (like > 4000 feet) I don't think it's going to be a problem.