I feel like I can do 12% off MSRP easy, I am going through internet sales at a volume dealer though not moving beyond ~10% (which I consider as an average deal in this market)
The rates just changed yesterday. MF is up to .00137 (residual 60% on the 540i, not sure if that was the same in December) - not sure if this was based on December rates.
Keep emailing other dealers and see what you can get. The 5’s aren’t a big volume seller so doing a custom order may limit how much you can negotiate.