The consensus seems to be that pruning back a paniculata too hard does not allow sufficient time for the wood to "ripen" or become stiff and therefore more self supporting. The suggestion is to never cut back any more than a quarter to a third of the branch length. This will allow the previous seasons' growth to firm up or stiffen and less chance for the flopping or drooping caused by heavy blooms on too flexible wood.

It is best to wait until the plant is dormant before pruning, so any time from late fall to very early spring.

Well, I guess I am in a difficult spot then because looking at it, I'm pretty sure I'm going to have to prune off more than that. And if I wait until the spring to do it, then I'm just going to have the same situation next year. Unless....I plan to take 1/3 off for the next two years, I wonder if that would work? Will it make any difference if I prune it as soon as it goes dormant instead of waiting for spring?