Thanks for the post. We often camped above 4000meters in the Cordillera Blanca and Cordillera Huayhuash, as well as other ranges in southern Peru and northern Bolivia. It was May and June, and your right, the weather was great. It is there winter, which is usually dry in the mountains, warm during the day and below freezing at night.
I felt like our tarptent did a fine job in the those conditions, and we weren't taking many risks. However, if we were in a different season, then I too would consider going with something more robust.
Where the tarptent was marginal was in Patagonia. The winds were constant, which means that dust came through our exposed mesh and coated our belongings inside the tent. The winds could be very strong, and the rain leaked through the sil-nylon on a few occasions. Southern Argentina (winds) and Chile (rain) are where a tarp tent is really testing your skill in tent craft, and your luck :)
What did you use in South America?