It's hard or impossible to know that delta. The amount of complexity that goes into rendering a modern scene is beyond what could be formally specified without putting nation-state level resources and decades of time into it. Even then, you'd have to go through a massive amount of analysis and development to find the most optimal implementation for a particular problem, and it would only be a local optimum, not guaranteeing maximum performance for the entire scene. Lastly, we don't necessarily know the absolutely fastest algorithms for any particular tasks that might be executed in rendering a scene. At best you could use statistical methods to guess at what the delta is compared to a likely optimum based on similarly complex scenes that are recognized as maximally optimized by known techniques (e.g. the demo scene)