Hi Bjoern,
I didn't had the time to play with the thing yet. But, from experience with other sims, this means very likely a 20% admission intake restriction (so less air to the engine --> less performance). But the impact will vary, depending on the car used.
It's different from added weight. In your example, +20kg means exactly that: additional 20kg (a fatter driver

) and nothing more. It may happen that on specific cars this will translate in more or less the same "penalty" in lap-time, but this would more a matter of concidence, and not an "as designed" feature.
Hopefully I should test it "soon" [emoji14]
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