What are the hash rates and temperatures you getting with 800W? what is the card's individual power limit?
I get 20.8-20.1 dual mining eth and decred at 482-485. Temps are 58-62 at around 75% power.
What is the dcri you use for this settings?
I get same rates & same temperature too (fans run at 30-35) with 68% tdp on Ubuntu though!!
just my 2 cents...