I've been reading through this post with some interest and bit of a smile. I have, for some reason, had pretty good luck with the species, including English, American, Korean, Japanese and the "tiny leaved" cultivars including Kingsville.
One thing I have seen over the years, is that most boxwood with the kind of substantial trunks that make great bonsai mostly DO NOT "want to become bonsai"--at least not in a single human lifetime. Boxwood are a bear to reduce drastically. Big trunk and limb reductions produce sharp, weird looking transitions between old and new growth that aren't easily or quickly disguised. I've seen people dig huge boxwood trunks just because, well, they're BIG. Not a reason to get one. I'd take a two inch trunk with lateral branching low down to an 8 inch diameter monster with
branching five feet up the trunk.
The boxwood I've worked were chosen carefully, at shows, nurseries and from landscapes. Bonsai-worthy boxwood trunks aren't common. Takes a while to find decent trees. Take a VERY VERY hard look at what you've acquired. Don't fool yourself that "I'll grow that top out" to complete the design. Most of the time, the "I'll grow it out" approach is frustrating and/or impossible.
