With ISI-based simulations, the problem is definitely not in the masses. Never was, unless people were/are really lazy and put in some mass without any modest research or common sense.
The problem has been with INERTIAS - chassis, engine, axles, wheels, brakes.
I have said so in the past, inertias are being, for the most part, wrongly calculated. I explained why and gave people some pointers on how to do it properly.
Obviously, some people prefer averages and percentages on averages for known inertias for this or that car, which is still wrong.
When properly calculated, the behaviour of cars changes for the better because the underlying physics engine is quite good.
From what I have been told, the problem remains the same with mods prepared for isiMotor2.5.