I did notice this summer, around 7700 feet (2350 m) on the Tahoe Rim Trail, that my SP Gigapower was a little temperamental, but at the mid-50 degree F (~ 12-15 C) temps I was in, this was an inconvenience of an extra 30 seconds boil time for 16 ounces (473 mL) of water. It simply wasn't a major deal and certainly not a matter of failure.
Conversely, at 9,000-11,000 ft (2750-3350 m), the burn rate was near normal. I was surprised by this, so maybe there is something to what the salesman offerred. However, this was an inconvenience at worst, not a failure issue by any means.
However, at sea level in mid-30's F (1-3 C), boil times increased by nearly 70-80%, and at one point I had to shove a cold cannister inside my down sweater in order to warm it up enough to continue cooking breakfast.
If I expect temps to consistently get to freezing or below, I'll still lug my Whisperlite, though I'm considering a switch to an inverted cannister stove.
For the Sierras this month, while cooking during daylight hours, I can't imagine the Gigapower being problematic at all. In truth, if temps mirror those of last month, you could boil water with no problem at the coldest moments of predawn.