Hi,
We purchased our P520 in Glenwood Springs while on vacation. We were staying in Leadville while vacationing. We got to enjoy the new Pioneer for about 3 days in the mountains and passes around Leadville and never noticed any issues with power. Of course, I have no frame of reference since it was brand new and new to me.
I looked at my Gaia GPS app, and it says we started our rides at 10,200 feet above sea level and got just short of 11,000 on a couple of rides.
We weren't loaded down at all, just me and my wife with two dogs in the back and a soft-sided Yeti cooler. I would say just short of 400 total weight.
Might be different now that I added bigger tires and a few more pounds to the P520, but I bet it still does fine.
Rob