I don't believe the slower casting time for cometeor will change anything. MID will still be covered with cometeor's every where. When you dodge one, you run into another then another then another. Cometeor is just too frequent, especially for what it does.

Strategy has little to do with it, since strategy itself is essentially absent in slaughter mode. You have kill the nodes that spawn at 11:00 more and go kill people at mid again. The only thing that will and can happen if nothing changes is that you will see entire alliances consisting of primarily BLM/SMN and healers.