The only 'safe' speed is the speed the miner had when sold. Even then an unmodified s9 is not known for being reliable.
When you overclock you are on your own and yes, you WILL shorten the miner life even more.
What do you operate your S9's at?..
I have mine adjusted up to 675M running about 14,300/14,500 TH they seem to be fine, low temps and 0% Diffa.. but its unknown territory as to will we burn boards out..or does board burning come at 15TH plus..
Your thoughts
Thanks mate
Jase