hi gaius - my wife underwent this processback in April and it was quick and relatively painless. I do not recall having to come up with any additional documentations that's not on the list. Just make sure you have everything on the list. One thing that slightly delayed us was making the payment over the counter on the same day of the appointment
, my wife had to reschedule another appointment. Otherwise it was very fast with no interview.
My wife may have more details with what she did.
Good luck and don't worry too much.