Upper limit of V1 is Vmbe (max brake energy) and lower limit is Vmcg. It can be moved between the two and it is generally constrained by runway length or Vr.
I think a pilot reaction time of a couple of seconds is built into V1 and most would not react well to a call of "Veeeonerotate!". Anything greater than a 3 or 4 knot split seems worth calling though.