No personal experience with this variety, but my experience with similar plants (Black Lace and Black Beauty) have led me to the conclusion that these beauties are not especially hardy. Both came back from the base of the plant after their first year, and then both winterkilled for me this past winter. So I doubt that you have to worry to much about pruning it back to keep it small - it might winter kill down to the ground every year! Sort of like a perennial, rather than a shrub. My Black Lace was a VERY fast grower though, in it's second year. It probably grew around 2 1/2 feet tall, maybe 3 ft in it's second year (starting at ground zero that spring). I think Thundercloud would probably be around a zone 4 plant, like Black Lace and Black Beauty.

Even though Black Lace did winter kill for me, I'm pretty tempted to try it again and just mulch the heck out it, because these are beautiful plants worth the trouble, IMO.