"Vigorous" boil?

Roughly, a back of an envelope sort of calculation.It's not linear according to tables I've looked at.A foot of wort is around 0.03 bar.
http://docs.engineeringtoolbox.com/documents/926/water_pressure_boiling_temperature.pdf

It means that in a brewery where the copper is several feet deep the boiling point at the base will be a few degrees above 100. Doesn't sound a lot but remember that a 10 degree rise more than doubles the speed of a chemical reaction.