The upgrade system still seems fairly primitive and I'd love to see upgrade files that, rather than just overwriting downforce/etc. values could also modify them by adding and subtracting from the default values. This would allow for much more sophisticated mixing and matching of parts and car customization.
What you're asking for already exists. For example, from the NSX upgrades file: HDV=[GENERAL] HDV=Mass+=5.92 HDV=Inertia+=(9.43,12.55,3.12) These values will be added to the HDV Mass & Inertia values.
