Any Naturally Aspirated engine will lose noticeable power as you increase altitude (in my experience). Air gets thinner, oxygen is more scarce. Same effect as with temperature, the hotter the weather, the more power you lose due to thinner air. Although you may accelerate easier at really high speeds due to less resistance (not sure on that one).
According to information online, at 6000feet, air pressure is at 11.1 PSI, which is is a 25% drop. I'd say that you would experience pretty significant and noticeable power losses.