Is a top heat gas grill, capable of 800°C / 1500°F perfect for pizza and sous vide steak?
No, at least for pizza.

Pizza needs to be cooked as fast as possible from all sides so that the crust is cooked and the topping are not over-cooked/burned.

In the case of a top-heat oven, you will over-cook/burn the topping before the crust is cooked.

i think it would be OK for broiling the steaks after vacuum cooking them.