Here is an update regarding my experience today in case it can help others.

1. LAX cannot open rentals for local walk-ins without the airport tax/fees
2. Very nice selection of cars - I ended up with an almost new Mercedes CLA, although somewhat stripped down in the features department
3. the manager was really nice, walked me around different cars, and then held that car for me while I returned my National rental, and then went up to the Venice location to open a new contract to avoid the airport taxes
4. everything was well coordinated - I picked up a minivan at Venice they wanted to get rid of, I brought it back to LAX and they put me into the Mercedes, re-printed my contract and off I went

I don't know why Sixt cut their daily rates so low in comparison with other rental car companies, especially for premium/specialty cars, but as long as they keep these rates, I am sold on Sixt.