I don't know about silkies, but in my experience the Ameracaunas would be over the fence in a heart beat! I have 4 Ameracaunas. 2 have taken to roosting in our shop either on top of the hanging lights or on top of the frame for the roll up garage door. Both spots are *well* over 6'. The other morning I walked out our back door and one of my Ameracauna hens flew off the roof of the house!!! That is at least 8', probably more.
So, I'd say you need higher fences or a top!
None of my other chickens push the height limits. I have mature Rhode Island Reds, Barred Rocks, a Jersey Black Giant, a Buff Orpington and then young Black Australorps & Black Sex Links in addition to my Americaunas. Freedom Rangers as well.... but they stay pretty grounded fat little meat balls that they are!
Liz