I just got back from using the Sigma 123-300mm f/2.8 on a Nikon D810 while on safari in Kenya and shooting gorillas/chimps in Uganda. Loved the lens, and was completely satisfied with how it focused by itself, and with both the 1.4x and 2x teleconverters.
After getting home I went into my local camera store to see/try the a7rII. Loving the 120-300mm, I had the store pull out a Canon EF mount version of the lens and the latest Metabones IV adaptor, and gave the Sigma 120-300 (without teleconverter) a go on the a7rII. It seemed the lens focused just as fast on the ar7II as it did on the Nikon D810. The focus was quick and locked on without any issues.
That said, please note that this was just a simple 10 minute test in the camera store. But, it's enough to 1) make me see if a good Nikon-to-FE adaptor comes out so I can use this lens, or 2) send my lens to Sigma and have them convert it to EF mount and then use it with the Metabones adaptor.
I'm waiting for the store to get Sigma's new 150-600mm Sport lens in stock, and then I'll test it out with the Metabones adaptor. I also did try the Sigma 24-35mm f/2 with the Metabones adaptor on the a7rII, and it worked very well.
All that's the long way of saying I think there are many long lens options for the a7rII.
Enjoy!
Jeff