Posted on Discord but not here yet, so:
Recently I realized fInitialDriveForce should be approximately N*m / kg.

Horsepower is a rather impractical measure, especially if talking about how a car is moved - and it's a function of torque and RPM. GTA doesn't do RPM, but does have a force driving the vehicle, or just the end torque - which is not dependent on wheel radius, so the value represents the torque applied at the tire. While horsepower increases proportionally with RPM, torque stays generally flat-ish. GTA's force at the wheels is (completely) flat (until 0.8 RPM where it starts dropping).

So this makes a case for torque. However!

It can be observed larger vehicles have lower values in order to accelerate slower, while you'd expect higher raw torque. Some sporty light vehicles have relatively large numbers, which don't coincide with what their actual output would be compared to bigger vehicles which should output a ton of torque.

Finally, plugging in the (peak) torque/weight number results in acceleration times that are comparable with actual data, and it's consistent for both large vehicles and small vehicles, either powerful or weak. It's not perfect, but remember many variables can influence basic acceleration - tire grip, drag, GTA's tire/suspension physics (curb boosting) and imperfect terrain. It's a good decent starting value though, for consistent realistic handlings.