I've just starting using SIXT this year as I have moved overseas from the US but still travel back and forth quite often. So far have had 3 rentals with an average rental time of 14 days and status matched to Platinum. On my third rental (first in MCO) finally offered an upgrade.

Location: MCO
Status - Platinum (Status Match)
Reserved - X5 (XFAR)
Offered - X7

Didn't take the upgrade though as the car was in another terminal and didn't feel like dealing with the headache and time.