The concept of overkill in RS is laughable, how does hitting higher then you need to slow you down at all? It doesn't, and from what I understand staves have a dmg boost to even them out with DW. Which, in theory is better because if you can take the dags down in less hits, you have new ones moving in faster. Imagine a dagannoth has 1500 LP, your spells do 1750 with DW, 3500 with Staff, staff attacks twice as slowly as DW. The extra damage that could have gotten to the next kill is completely wasted when you are using the staff. This is an exaggerated example, but the point is still clear, slower attacks at the same dps means the damage is not as spread out as evenly, and hence will always result in slower kills because much damage has gone to waste.(or is more likely to go to waste). If every dagganoth was as hard to kill as QBD, then staff is of course at least equal with DW. Not with dagganoths in current state. Staff is guarenteed to kill slower than DW of the equivalent tier when it comes killing things that have low HP compared to the damage you are dealing out.