I have an Xperia Z4 and it's the hottest phone of 2015 (literally).
It's so hot that Youtube and the Camera app closing on me became an everyday thing.
I figured I should reseal the back plate so that waterproofing would work again (with a "back cover" seal sold online). However, I'm also thinking about putting a few layers (or a slightly thick plate) of copper and applying thermal paste/pad over the plate covering the Snapdragon SOC.
Of course, to keep it safe I'll be isolating a few parts of the copper to avoid shorting, but I wonder if this is a good idea? Gonna sell the phone but at least give the phone a bit of hardware fix so it won't toast the new owner's hand.