Some interesting reading
11. However, under the terms of the lease the full capabilities of the C-17 will not be available and the aircraft can only be used as a strategic long-range transport, albeit with the ability to land on short runways. The C-17s will be restricted in operational use and their capability for para-drop, airdrop, rough field, low-level operations and air to air refuelling will not be used. It can reasonably be assumed that this is a restriction imposed by the lease conditions, or because the airworthiness of the aircraft in these roles cannot be evaluated by the DPA in the time available. .
http://www.publications.parliament.u...36/1011719.htm
How much is ture, and whether that will change once we own them is, well, hmmm.....