Arborvitae are often kept smaller than their natural height. Most often this is done as they approach the desired height. Cutting that deeply into the canopy will leave you with many unfoliated stumps and will look pretty bad. Having said that, Arborvitae will sprout new buds near the stump cuts as a result of the heavy pruing. Growth will resume at the point of the cuts and in a short time be taller than desired. There will likely be a loss of some of the root sytem as the canopy and root system are in balance right now. If you can prune in such a manner to create a rounded top, the regrowth will look at more natural.
