YUP. The higher the altitude the greater the HP loss. I can't remember the figures but someting like 30% to 40% loss at 10,000 feet. IF that is correct, and I am not sure that I remember it properly, you are at 85% of 10,000 feet so you'd lose 85% of roughly 35% of your total horsepower.
The odd thing is that with a Turbocharged diesel engine, the effects are virually eliminated.
Regardless of brand you choose, at 8500 feet above sea level, I would recommend you pick one with a turbo diesel. That or buy a lot bigger tractor and realize you may get less than 70% of the power the engine is theoretically capable of producing.