As the title says, the game wants us to upgrade cars but even at similar PI road cars are often beaten by actual race cars as they are custom built for the track.
Adding the below to production cars could address this and make racing production cars a lot more fun:
Stronger aero
Wider tyre options
Track width upgrades
Fuel cells/smaller tanks
Smaller rim sizes
Transmission upgrades with more than 6 gears
Boost by gear forced induction tuning
Customisable restrictor plates for all cars
And not a performance upgrade but pod gauges or race dashboards would improve immersion