Hi, since i saw this post i made every type of mod, using only thermal paste (MX-4), thermal paste + thermal pad, removed the metal lid and changed the thermal compound (that pink crap), and always got the same results.
Today i performed a little modification inserting a piece of cooper plate 0,5mm thicker between the lid and the metal body, see the photos below:
Google Photos Album if the img above fail:
Applied some MX-4 between the lid and the cooper plate and the cooper plate and metal body, just a little dot right in the middle.
Between the SD810 and the metal lid, i used a GELID Extreme ThermalPad 0,5 thick with 12W/mK
And, the conclusion.... i got pretty much the same results (when i take a picture with NX Camera or GCAM reaches 70 Celsius in a few seconds, so nothings really changes. I just think the TDP of SD810 is too much for Nexus 6P handle it, especially without a heatpipe to help.
Anyway, that's my experience with this mod, in a daily use i configured a power save profile to help and prevent any thermal throttling... i think im going to forget about it.