I put in quite a few flights on 428, 422, and FAI at about 7500 feet in Mexico City about ten years ago. All three classes ended up with essentially the same setup at used at normal elevations. Yes, the engines made less horsepower, but the props had less air to bite into so it evened out, with RPMs in the same ballpark as at home. Lowering the head seemed to do very little if anything Bob (HighPlains) said, it's not too critical with sport fules The only real difference was takeoff distances were increased, as were turning radius and landing speeds.

More recently, I flew a couple Q40's (Nelson LS) at 4300' field elevation a couple years ago, with density altitude around 5000'. They ran just like they did at Florida, Phoenix, and LA with no changes.