So you agree that both Forza and GT are flawed 'racers' because I've already mentioned how unrealistic the 3-5 laps setup for the majority of the games is.
You're missing the point. If they were lazy enough to pull graphical assets from a previous generation game in order to release it (and to claim it has 1000cars), do you honestly believe they might not have been lazy with other things such as the physics for those? One who is lazy with one thing is surely lazy with others. Nailing proper physics takes more time and effort than generating a computer model.
My logic is more grounded than you think...
IF GT5 is doing any form of proper real-time aerodynamics, the blocky Standard model cars WOULD be affected by this. The simulated air would not flow as smoothly over these blocky models as it would over the nicely curved premium cars. Good lord, the tires are even somewhat blocky for some Standard model cars (we are talking about PS2 models here). This could make one start to question the real-time tire physics as well and if there even is real-time tire physics. Have you ever tried to roll a Hexadecagon? It surely does not roll the same as a more circular object would....
Therefore, either GT5 does not do real-time aerodynamics and tire traction, or if it does, the physics model would without a doubt be affected by those graphical assets being used; take your pick.