Not directly related to the application in a VW, but I know a guy who has air bags insead of shocks in his HSV (rears only) and he swears by them!
I don't know the details but I know he can easily adjust the stiffness for different applications. Cruisinng, track or towing his boat.
The best way I can think of to approach something like this is let people know what your goals are and then work out which solution would be best for what you want.
Cheers
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