Answer: From experience I've found that summer pruning results in rampant growth with Japanese maples. This is good if you need to cover up poor pruning cuts, but not so good if the excessive growth obliterates the desired shape and form of your tree. If it were my tree, I'd go ahead and thin it out now, in the winter when the tree is dormant and you can see the branching structure clearly. Prune it back to a more manageable size and shape. Then, in the spring, new growth will appear and you can rub out any new buds that threaten to grow into misplaced branches. Best wishes with your Japanese maple!
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