So why are taxes even progressive for the 99,99%? They achieve just about nothing in reducing GINI, they piss of the upper-middle who may be unable to buy a nice car...

The purpose of progressive taxation is not to reduce the Gini coefficient; it's to efficiently extract funding and to sound good to fairness-minded voters. With regard to the former, there's a lot more people around the 90th percentile than the 99.99th, more of their money comes in easily-taxable forms, and they're generally more tractable than those far above or below. They may be unable to buy a nicer car after taxes, and it may piss them off, but they're not going to be rioting in the streets over it, and they can't afford lobbyists or many of the more interesting tax dodges.

With regard to the latter, your average voter has never heard of Gini nor met anyone truly wealthy, but you can expect them to be acutely aware of their managers and their slightly richer neighbors. Screwing Bill Gates might make good pre-election press, but screwing Bill Lumbergh who parks his Porsche in the handicapped spots every day is viscerally satisfying and stays that way.