"...in the US they arent growing very fast" and are "somewhat spindly."

Since we don't know where you are or what the conditions at your location are, it's hard to say why your trees aren't growing well. Generally, in locations where they do well, they are known to grow very quickly (one of the fastest growing trees out there).

"no woolly adelgid problems for leylands"

With bagworms, various scales, spider mites, Seiridium and Botryosphaeria cankers/dieback, and root rot disease (and many of these pest and disease problems intensified with the more and more frequently seen monocultures)...who needs adelgids?

"In order to get a more robust bushy look does it make sense to prune the branches back a bit?"

no!