I think I'll step in to end this discussion here.
From a game standpoint, we have to keep the cars running at their approved levels at the start of all single player game modes. The main reason that 'Equal Cars' is available in Multiplayer is so players can have a fair experience. Without this, the game becomes impossible to race on for a lot of users.
Although its a nice idea, having F1 cars all run at equal performance in Grand Prix mode would not be truly indicative on modern-day F1, and would be very difficult to persuade all stakeholders that this would be viable.