Downforce

[Snix]

Body shop parts should remain visual only.

They should and I suggest they do. They should come as VISUAL ONLY and at the normal car's performance and ONLY if the player chooses, he can adjust the downforce in the dyno or some special menu to change it.
That way it's visual for those who don't want to change it and to those who like fine tuning they can fiddle around a bit
Of course you can't change the speed and grip values of a car massively with just a spoiler but there should be some noticable difference once you add the downforce value.