This tree is by far and away my most vigorous juniper and usually needs significant pruning several times a year to keep it in shape. I cut it back hard for the show, as I wanted a sparse canopy...I honestly don't know if I removed exactly 1/3 of the foliage or not. I removed enough to stress the tree out, though...it sulked for about a month after the show then started pushing Juvenile growth EVERYWHERE. It reverted back to mature foliage late last summer and continues to grow strongly. Anyway, I am planning to graft shimpaku on to this tree...maybe this week.
Fwiw, I have removed WAY more then 50% of both a juniper's canopy and root system (maybe not at the same time, though, but I've come close). I've routinely done this working down nursery stock. If the tree is healthy, it's the right time of year, and your horticultural practices are sound, you can be very aggressive with pruning and not run the risk of losing the tree, or even slowing it down.