Cordierite can withstand extremely high temperatures. The pizza stone is designed for use up to approx. 500° Celsius. It loses its warranty when used at higher temperatures. If you still want to use it in the top heat grill, make sure that you heat up the stone evenly and allow it to cool down.
According to the pizza stone manufacturer, you can even place the pizza stone in the oven for cleaning and select the pyrolysis process. That is 400°C. I only use the pizza stone in the oven. It is important to always put it in a cold oven and only then heat it up, and the pizza stone must be really hot before you put the pizza on it to bake. Allow to cool well after use.