There is definitely a power loss at altitude, just comes with the environment. If you are coming from sea level you will notice the loss, but the car will still run great. Live at 6500 in CO and on my weekly commutes I travel +/- 2000 vertical feet. On weekends I head up to the mountains, taken the WRX over 14K feet, Mt. Evans, Pikes Peak and many back country passes.
Trust me you would definitely notice the power loss of altitude in an NA car a lot more than what we see.
The turbo is great for providing power when most cars have none.