@BOAC, the EFB tools i know start with a take off performance calculation and for that need a take off weight as entry condition. Our EFB is set up so that it uses by default a value well above MTOW (86t for the 800 and 81t for the 700) so that the max take off weight calculation is not hampered by some lower value.
Engine ratings are usually chosed as the lowest possible one for any given weight (hence Optimum), however with a weight above MTOW the EFB should revert to max rated thrust in most cases to make a departure possible.
Landing weight calculation has to be done either with another tool in our case, or even more comfortable by the centralised load control as they do know the maximum landing weight and fuel burn, no need to give them those figures.
Thrust rating is something that is different for each single departure, therefore it is of some concern, for a RTOW calculation however a well written EFB tool will try the highest possible one to get the highes load possible out of a field (remember, in the contaminated case it might well be a lower than the max one for an increased V1 margin).