I was extremely interested in UCSF because their research is very strong in the area where I've done a lot of research. I did all my aways on the west coast and my application was fairly clear that I want to be on the west coast. My supplemental application for UCSF included a first-author, high-impact journal publication, and highlighted specific investigators at UCSF I knew, whose research was similar to mine, and with whom I'd love to work. I probably should have awayed specifically at UCSF, but alas, I was trying to save some money so I went to another, cheaper locale, west coast program.
I took step 2 CK very early in part because my step 1 was about the national average for rad onc and in part because I knew UCSF would be a top rank for me. I knocked step 2 CK out of the park.
Alas, UCSF didn't invite me to interview. There are no other programs that require the score to rank. So I kind of wasted my time. So after all that, I wouldn't recommend taking step 2CK for UCSF. If you do end up getting an interview, then think about cramming it into your November, December, and xmas break time to get the score back for rank list time. This is what a buddy of mine did (mostly for UCSF) and he scored very well.