It doesn't seem like there was much in the way of a warhead though (didn't do much when they did detonate), and it seemed like they didn't have an exact fix on Khan, rather just an "area" to bombard. Khan's beaming location was verified, and it was time-sensitive for how long he would be in that area, so they'd saturate said area with warp capable missiles outside of threat space, most likely killing Khan in the process.
For that you'd need more explosive power than what was show later on. Khan could have replaced the warheads with people. That ring-sized explosive would be quite potent when increased in size, say about human-sized. That'd be more in line with saturating an area.