I was up over Loveland Pass several times in the last few weeks and going back that way next week. I'm there more in the winter than the summer, and I think it's a scarier drive this time of year as the contours of the drop-offs are more visible and you can really see how far down it is all around you. When it's all white, it's hard to judge the distances.
The electric drive loses no performance to the altitude, except for some loss in cooling efficiency when pushed very hard. The regen on the descent regains most of the energy used on the climb. The steeper the descent, the more the loss in energy going up and down, but even relatively steep descents like the west side of Wolf Creek regain most of the energy. If above about 70% SoC, the sustained regen can run into the same charging limits as on a Supercharger; I have hit this on the east sides of Wolf Creek and Independence after leaving with 100% SoC from Pagosa and Aspen. Normally short bursts of regen never show a regen limit below 95-97% SoC.