If you have a turbo charged car that is running loads of boost through a poor intercooler (ie a chipped calibra turbo for example) its not uncommon to massively exceed the rated bhp figures for the jets just thanks to the colling effect.
Also at some points in the rev range you will see MASSIVE gains from spooling the turbo earlier.

Olly clarke for example made 300bhp at the wheels extra on his scooby with 100bhp jets at about 5K rpm.

Basically its very application specific and no general rule of thumb will work for all cars.