If you mod the Bios they are out of warranty.On the other hand you get 25-26 Mh at standard and 29-30 Mh at modded Bios.It is a preferance.
I had problems with a RX 470 that was strange and crappy from day one.
I flashed the card and later on it just died.
I returned it with flashed bios and got a new one the same day.
If a gpu is dead it is dead, nobody will put hours of work to figure out what bios is on the card.