Most of the time you won't notice. But you will on mountain passes over 7000 ft. Don't worry about it just go for it.
All naturally aspirated engines loose about 3% of their power per 1000 feet of altitude. Sorry guys, that's just physics. Ok a few Venture riders are exempted from the laws of physics. But for the rest of us these laws apply. Noticing the loss depends on how close to the upper horse power limit you were riding. In other words the further you had the throttle cranked the more noticeable it is. It is most evident when passing and on grades. That said many Venture riders never notice the change in power at all.
How you can tell if you are being affected. 1. The cruise will kick out on small grades. 2. The engine will feel like it is lugging in 5th even though you are at speed. Especially on a grade. 3. Passing or accelerating is sluggish, requires more throttle.
Premium fuel it harder to burn than regular. So don't switch.
Mike