Am not overly familiar with Thuja Green Giant, but if I wanted a hedge to 15', something with "Giant" in its name does not sound like a good bet.
Carolina Sapphire appear to take to hedging easier than some trees (not sure how they would do in your area in Z6), so it probably is possible with some trees. However getting up to 15' to prune seems like a daunting task. Perhaps Northern White Cedar, an Arborvita of some type, or some Blue Point Juniper would be a better choice. Will see what those actually from up there recommend.

no tree stops growing.. because you top it ..

it will releader and go skyward ..

as scot notes.. it has way to much potential for your project ....

and i strongly emphasize his comment about how you maintain anything at 15 feet ... that is 7 feet above a common ladder.. and unless you are nine feet tall.. and standing on teh top of the ladder .... i dont know how that all works out ...

get rid of them ... at 15 feet tall... they will be near on 10 feet wide ... do you have that much space at the bottom???

i suspect you got sucked in by the marketing hype.. that they are fast growing ...

ken

Keeping them pruned to 15' would not be easy at all. Topping them will stunt their vertical growth for like a whole day.

Haha, thanks for the straightforward answers!! Yes, I got sucked in by how cheap they were and how fast they grow. I may keep one or two and put them in a different area and just let them grow. Thank you all for the reality check!