hello I also own a fornetto alta and contacted the support team about the temperature of the oven. They said it isn't a problem to exceed the 400 C but not for hours.
I made many pizza's around 400 C in the oven this year and did not have any problem with the oven. It is a bit dificult to get the heat long above the 400 and the stone can get to hot so the pizza will burn at the bottom
Have fun making pizza's