(1) Mainframes require purchase of SR5000, SR5020, or SK5010
I'm pretty sure you are correct, the new SR5020 has an MSRP of 11,800. I can also remember in the past the the SR5000 was needed in order to connect the BK5000 because I got burned on this when I did not read the release notes.
There are no release notes next to the SR5020, so I would have to assume that all you need is the SR5020.