Well, there are some trade offs involved with adding more than 250HP to an already highly boosted engine. The current practical limit for turbocharged engines, while maintaining warrantable reliability is about 2x the displacement in liters. That’s around 700HP for a 3.5 liter engine, which is about what the current NFGT produces. The McLaren 720 uses a 3.8 engine, and the high end 911’s have 3.8 to 4 liters, and close to double that in HP.
Higher numbers can be achieved, but at shorter MTBF.