So I found what I was looking for, right in the game menu. Can't believe I overlooked it at first.

Deadzone Removal Range it is, and it should be quite high for the G27 in order to bring back the detail (since the FFB stays dead up to around 18% input). I started with 0.18 and that brought back some of the detail, but still very shallow. So I upped it to 0.25 and now it feels great. Higher than 0.30 is not advisable as this will make the FFB motors work against eachother when going straight (oscillation).
So my current settings, changed from the default are:
FFB Strength: 100%
Tyre Force: 150%
Deadzone Removal Range: 0.25
This setting actually feels great with road cars, brings back the missing detail. Kerbs, grass, small bumps, you can feel it all.

There's still some FX drownout when turning hard left or right, but I'm sure I can tweak that out.

Will keep you guys posted on further testing and also with different car/track combos.
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