Actually, I don't know if this quite answers the question. Is it more efficient to make all objects on a layer the same blend mode as the layer?
For example, if I make a layer additive, but all the objects are blend mode 'normal', the objects on the layer 'add' to the layers below, but not to each other.
However, if I make both the objects AND the layer additive, the objects add to each other, in addition to adding to the background.
Finally, I can make the layer 'normal' blend, and all the objects 'additive'; in this case, the objects add to each other, but the layer itself is blended normally.
So, really, it's three different effects. But...which is the most efficient? I'm tempted to think there isn't really much difference, unless the objects themselves have different blend modes from each other.