Stanley Eevil, - “…performance manuals assume a 15kt overspeed…”
I believe that you are perpetuating a widely held misbelief; see:-
Landing performance of large transport aeroplanes.
Modern certifications/operations use a factored landing distance:-
“…in order to provide an operationally realistic value of performance, this gross distance is multiplied by a field length factor to obtain the Landing Distance Required.
This factor accounts for the normal operational variability that can be expected in day to day service such that the chances of a landing overrun are remote”.
The AIC does discuss an (old) alternative method of determining landing performance from a ‘fast landing’, Vref+15 (and from a low screen ht, 30ft), but this ‘error value’ was an alternative means of contributing to a safety factor catering for operational variability. The speed error, or field length factor, are not margins to be employed routinely without thought about the change in safety / risk.