The F1 turbos were restricted in displacement only. Turbo inlet sizes and number of turbos were unrestricted. Nobody really knows exactly how much power those animals made....the Renault dyno went off the scale above about 1400 and they still had 1000 revs left on the tach.
A 32mm restrictor has enough airflow capacity to support just north of 300 hp at the high end of flow. HOWEVER, it supports quite a lot of torque well below the peak power level. This is one of the reasons that they run anti-lag on the cars. There are other reasons as well.
BTW, the 300hp (ballpark) figure is NOT a 'gentleman's agreement' it is a physical limitation based on CFM of airflow through the restrictor.