No need for a speed / time check on 777 as ASI has a speed trend vector incorporated in its display. Effectively it is an obvious indicator of your level of acceleration during takeoff. On 777 it will usually extend 30 knots. I would think the Airbus would have similar.
Assumed temp method of calculating reduced thrust take-offs is the most conservative method because the various takeoff speeds it provides assume conditions are more restrictive than they really are. Full thrust can always be selected after V1 without fear of being below Vmcg.
Its very simple in practice and not a factor so long as correct data is input.