I was on an XCAR booking last year in September for an 18 days rental. Got assigned a Volvo S90 T5 with a flat tire. Mentioned this to the Sixt people and was bumped into a 3 row XC90 T6. I was happy with the move.
They tried to upsell me on some kind of toll-pass, which I repeatedly had to decline. If I wanted, for a price, I could upgrade to a GLS, a GLC or an X3 (this was before the flat tire put me into the XC90 free of charge).
To note: I do not and did not have any status to speak of. I'm not sure being platinum will get you upgraded beyond XFAR, though I guess it depends on the person you meet at the counter.
The guy driving the shuttle to the rental facility was a hoot, and the line at the counter was ok (or maybe non-existant). Drop-off was eventless.