For the heads-down time, does it really require more interaction time than paper strips where you have to write and sort the board manually?
Yes, very much so.
My study did not see this as a factor once the ATCO learnt how to interact with the HMI efficiently, which shouldn't be more than a matter of a couple of sessions in a simulator.
What efs system did your study use, and was this in an actual aerodrome control environment, an actual centre environment, or a simulator environment?
What light levels were present in the environment the study took place in?