purely annecdotal, no power numbers, but last fall i raced mt evans here in CO starting at 7500 feet and climbing to 14000 feet.

i got 8th in 4, with a time that woulda put me mid pack in 3, totally unexceptional.

maybe a month later, in similar form, i did a grassroots version of the Mt Charleston Hill Climb outside of las vegas. similar grade. similar consistency, just considerably lower (3500-9000 feet) the real race got cancelled but a number of folks showed up and did it anyway, and i crushed it, took first over a number of 3s and 2s who considered themselves strong climbers. it was hard to compare finish times to previous years cuz it was quite windy, so i dunno how worthwhle that is, but i did quite well and moreover, whenever i ride/climb/race at lower elevation (i live at 5000) i FEEL strong. the air feels rich and thick and my legs feel strong and like everything is firing well.