I never noticed HP loss at altitude... my tiny trailer is far too light for the hemi to notice it by weight alone.
what it does notice is a head wind. I forget what highway it was, coming out of Vegas toward death valley, a really long moderate grade climb to only about 5000-6000ft? STRONG wind from the West, as I was heading west. This was the only time I've taken pitty on the hemi and backed off from the speed limit as I watched the oil and trans temps climb into the upper 25 (percent) of the gauge.
I don't have a manual transmission, I don't pretend to have one either, playing with the gears. I occasionally downshift on steep downhill grades when tow-haul doesn't for some reason (usually it does automatically)
just my 5cents
oops, forgot good sam inc. doesnt like percent signs