Taking the liberty of knocking of 3-5 kts from your performance calculation, to my opinion defeats the whole purpose of calculation.
If you move your V1, it will help you to stop, but it won't help you if you continue your takeoff with an engine failure that could have happened between that -5kts and V1. So what do you do when you are obstacle limited but have enough runway in front of you. Do you call V1 after V1 ?
I'd rather get off the runway while on the ground, than hit the building while in the air.
These procedures go against the whole performance calculation that was designed by the builders engineers, and give the freedom to airlines and pilots to interpret a given situation to their own standard.
Where do you draw the line ?