Equal header length only aids one small rpm range. Exactly the same as the intake, that is why we have dual runners on our cars (sorry zetecers). Mazda once had a valve in their exhaust to give it dual header length. These benefits will help all cars, turbo super NA. But on forced induction you will not notice the difference.
As far as the spooling, this is determined by two things turbo mass and pressure. Short headers give less volume thus faster spool up. But do the math, >2.5 liters per rev, volume of the headers is going to be much much smaller, so this should be pretty insignifigant.
In and ideal engine your would have trombone pipes on the intake and exhaust that varied infinately from 25 feet to about 5 feet. I think I heard that variable intake could increase your map by up to 3 psi, but only at a specific rpm. BMW M3s have an infinitely variable intake, but we have no room for that (maybe on a zetec).
Take this with a grain of salt, I only speak from my limited experience.
