From memory the Twotter has a ferry weight of 17500lbs and it is in the flight manual as a supplement. I have on more than one occasion seen a ferry weight Twotter take off. It is a curvature of the earth take off.
One time I saw a Kenn Borek otter come in from a ferry with ferry tanks, 20 litre fuel drums and other gear, I am sure the ferry weight was a healthy 17500lbs. They are an aircraft which over the years have been used and abused but have stood up to it. I have pics to prove it. They are one of my favourite aircraft.