last rental in MIA I got upgraded to full size SUV suburban and they told me they blocked a 1500 $ deposit on my cc for the duration of the rental.
I knew about this blocking of deposit beforehand as some other rental firms in Hawaii sometimes did the same to me without any warning and so I had already requested a higher cc limit before leaving for my vacation .
I read on a few other FT-posts that sometimes the deposit can be lowered by the local sixt staff -with approval of the stationmanager- and this could also be depending on sixt status ...
Maybe it is best to get your question forwarded to the Sixt team following all posts here on FT ( Gary and other UK based sixt colleagues ) as they sometimes can get you specifics depending on where you plan to rent or even get things worked out before you arrive at your destination !