First, the local Sixt branch has resolved the smokey car issue. They offered me a complimentary rent to compensate, and I accepted. Need to haul some stuff, so a van to do so for free is a good deal.

Regarding upgrades:

Berlin Tegel
Sixt Diamond card

Booked: FDMR (Bmw 3er or similar)
Received: FWMR (Audi A4 Avant, 170 hp)

Car wasn't too bad, but I had hoped for a slightly better upgrade. Branch quite sold out due to a fair in the city. Tried to haggle my way to a Jaguar Coupé with no success. Oh well, maybe next time.