I'm at 8500ish feet. I've only ever soaped at this altitude, but I do think I have to use a little more water in my HP than other folks to achieve the same fluidity. My crockpot is a newer model though, and I think cooks at a higher temperature. It's also pretty small which I think makes a difference.
I've done M&P at sea level and up here... there is a noticeable difference with that. The same base gets a skin much faster up here. Though that might also be due to the difference in humidity between Houston and Colorado...