Yep, excessive heat kills cats at any HP level. At the OEM level we try to keep cat temps under 950 deg C. Most tuning shops do not use temp thermocouples though so a lot of times it can be a shot in the dark because cat protection is modeled in the ECU. I have seen A/F as rich as 0.70 lambda to keep the temps down and I'm sure there are some applications that will go even richer.