Coaches all the time say run as a group and work together but if your top five guys are running 15:x at the top end and 17:mid at the slow end at their best, what the hell does it mean to run as a pack? They can't run as a pack, so why tell them to run as a pack?
It will feel like practice and that will feel comfortable and your runners will be more relaxes. Also, it keeps people from going out too hard and blowing up (except maybe your couple slowest guys in that pack) and this reduces the risk of having someone that you really count on from doing terrible. If you pack run the first 2/3 of the race, then your top guys should still be in contention to pick it up a bit since they should feel somewhat controlled, and then your slower guys can just try to hold on at that point. The alternative is your top guys go out pretty quick, one of them blows up, and then one of your slow guys also blows up because they were going to blow up anyway. Negative splits are way better than positive splits, so pack running guarantees negative splits for your top few guys. You just don't want them to be TOO far negative.
Your top end should pack up, and your back end should pack up. 15 minute guys together, 17 minute guys together.