Overkill? I guess it depends on your goal. You can know Step Up cold and you won't get every medicine question on the test. But for the most part, yes, Step Up goes above and beyond the detail that is required for CK. After looking at Secrets, I think you have to pick and choose your spots with that book. Put it this way, if you are going to do all of UW, you probably won't find much (any?) IM info in Secrets that isn't already covered better in UW. On the other hand, Step Up will give you extra info that isn't in UW.
I used Secrets more for things like ENT, Ophtho, etc. which aren't covered all that thoroughly in UW.
But seriously, if you know everything in UW, you will score above 260 quite easily (not that knowing everything in UW is easy, or even possible). So all the debate about secondary sources is kind of moot unless you really just have to go that extra mile. If you really drill UW hard, whatever books you use on the side are going to work fine.