You are not dropping 20kgs, so to use that as a reference is silly.

You are not driving F1 cars..so again, silly. I'm sure the Volvo 940 in question is not 500kgs...so again this is silly.

There is no way in hell on your average weight road car you will be able to measure a 20kg weight difference in terms of real world performance.

I've raced mine with passengers and it's made zero difference....on some runs I even went quicker. And that's a 100kgs change. Obviously that wont apply to all, but it's fact for my car.

People can get carried away with the most irrelevant things. It isnt to say losing weight can help, of course it can...but be sensible and practical about it.
A decent reliable battery can be around 10-12kgs...a mega expensive race battery that may not be so reliable will be around 2-5kgs and very expensive to mega expensive.

Really...is it worth it for a real world 5-6kg saving ?