I've flown at 2,000 feet in 50 degree weather and at 7,200 feet in 100 degree weather. There is a huge difference in performance. Once you get used to it, it really isn't that bad.

I fly for the most part between 4,200 feet and 7,200 feet. The density altitude goes up to 7,000 to 10,000 feet. That is the reason I step up to the next size engine for my planes. In a lot of cases we void the manufacturer's warranty of the plane to power it like it would fly at sea level. I once talked to a support tech from one of the big companies. He said that the planes were tested "at altitude" for performance, I asked what "at altitude" for them means and it turns out they considered a density alititude of 4,000 feet "at altitude." Since the majority of buyers of their products reside at lower altitudes, not much research is done for the higher altitude flyers.

I have noticed that at lower altitudes the prop engine combo's that I tached will turn at a slightly higher rpm. For the lower altitudes the bigger engines are not a problem. You just need to learn how to manage the throttle stick. It is also nice to have extra power there when you need it. There are too many WOT flyers out there. Just look at how often a full scale plane is flow WOT....

Here is a calculator that computes relative power:

http://wahiduddin.net/calc/calc_hp_dp.htm

If you really want to see what happens, take a vacation to a higher altitude field and go flying. You'll notice that you need a little more runway to take off, a higher throttle setting to hover, more speed or more rudder to hold knife edge, and a higher speed when landing.

I own a power boat and there are a lot of lakes around here that you can drive to in about an hour and a half from where I live. They vary in altitude from about 4200 feet to 7500 feet. You can really see a difference in performance depending on which lake you go to. It would be interesting to take the boat down closer to sea level to see how it handles, I wouldn't know what to do with the extra power.