So, I installed the Hubbell. Yup, it makes a difference. It is only 54 degrees right now, but the plug has been pulling 5kw continuously for 3 hours now and the outlet is in a brick wall with a cover so I doubt the mild outside temp plays much of a role. I unplugged it and held the prongs in my hand and could not perceive any warmth. Probably it was warmer than ambient, but with the Leviton it would have been uncomfortably hot to hold onto.
I do have one of those IR thermometers, but it was not handy at the time and I already un-installed the old one.
I do have one of those IR thermometers, but it was not handy at the time and I already un-installed the old one.