The pcb gets warm at 1.6v and hot at 1.65v. You need some airflow aimed at the sticks at higher volts. The pcb these are on is simply not designed for higher voltages, the chips will take it fine though.

I wouldnt run over 1.6v without airflow and 1.65v with airflow 24/7. Benching youll be fine up to about 1.75v where they stop scaling (again airflow required).