I don't refer to the "benefit" to the turn rate, rather there is a bug where full flaps (in any aircraft) increases the drag massively, acting as a huge speed brake. This has been admitted by HTC (Pyro, or Skuzzy, or somebody admitted it).
The lift is still there, I bet, so the help while turning remains, but the turn performance is also hindered greatly by slowing the plane down well past the "stable" point.
That's the bug.