I dont quite agree with calling V1 earlier than calculated.
The reasons being, firstly i believe that manufacturer has done enough tests to determine the correct speeds and have put it in the Perf manals.
Secondly the whole idea about callling V1 earlier seems to be to avoid rejecting take off above V1 and there by causing an over run, if we keep in mind the following things it would help
1) Ef occurs before you have heard the call V1, you have the option to reject.
2)Ef occurs at the moment you hear the call V1, not much of a choice there but to go, because by the time you react to it you would have crossed V1.
3)Ef occurs after the call V1, by now your hands are already off the thrust levers so the decision has already been made.
V1 speed takes into account the reaction time, therefore by calling the speed early you are committing yourself to the takeoff earlier than you have to.