Just got a reply by email from Sixt US customer service about deposit:

That is correct for a luxury line car our deposit/hold does not go below $2000 USD on a credit card. On cars up to full size the deposit is just 3 times the amount of the rental.

So Sixt gives amazing rates but puts hold for amount that can't be justified by any means given that all the insurances are included in rental price. For me this policy makes rental with Sixt unthinkable and I'll cancel my rental and switch probably to National. Got $350 rate from them with KLM/AF card for LCAR / 8 days in MCO. Sixt would be probably great idea if you rent economy and don't care about deposits and have a short term rental.