Japanese Maple leaves can look a little tired this time of year, unless a fresh new flush of growth occurs. For your zone, you would need to protect the roots in winter, so if possible i would put it in the ground now, without doing much root work besides a light outside trim. In late winter, long before the buds start to swell, completely bare root the tree and do your root work and cut back of the top. and put it in a low wide container, tied in, with professional bonsai mix and let it grow. If it grows well, you can begin to wire after the first leaves harden off beginning of Summer.